The plea came from Leah Likin, a fifth grader. It was addressed to her mother, who had registered both of them for a two-part course on puberty called “For Girls Only.” The missive, which included additional objections, failed: Mother took daughter anyway. But Leah had plenty of company, peers who shared her resistance, their arms crossed, their eyes downcast. Last year, the course, which is split into sessions for preteen boys and girls and held mostly in and around Seattle, and also in the Bay Area, pulled in 14,000 attendees. They heard about it from their pediatricians, or through word of mouth.

The creator of the course, Julie Metzger, has been trying for nearly three decades to turn what’s so often at best a blush-inducing experience — the “facts of life” talk — into a candid dialogue between parents and children. In the mid-1980s, she was a graduate student at the University of Washington School of Nursing when she reviewed survey data on how women had learned about menarche, or the onset of menstruation, for her master’s thesis. Most reported getting information from gym class or their mothers. “You can picture those conversations lasting from 10 seconds to 10 hours,” Metzger says. “And I thought, Wouldn’t it be interesting if you actually had a class where you sit with your parents and hear these things from someone? What if that class were fun and funny and interactive?”

Metzger, who is 56 and vigorous, with flushed cheeks and blue eyes, says she has always been comfortable talking about sexuality; her father was a urologist, her mother a nurse. “Hand me a microphone,” she says. “I get so into this topic that I can make myself cry in front of the class, and it’s real.”

Her class on puberty debuted in 1988 at Allegheny General Hospital in Pittsburgh, where she was the nurse manager of the pediatrics unit. The class was so crowded, she says, that “we had to run it twice.” That reception convinced her that there was an appetite for a forthright talk about growing up. Soon after she moved back West in 1990 — she was raised in Portland, Ore. — Metzger began offering the course at Seattle Children’s Hospital.

“Parents walk in feeling almost victimized by preteens and puberty, and my job is to utterly transform their ability to connect,” she says. “That sounds so arrogant, but I know when I walk in that room, that is my work.”

On a recent winter evening, Metzger stood at the door to the hospital auditorium and greeted every mother-daughter pair with animation, as if she’d known them for years, and told each girl to take an index card and a ballpoint pen with the name of her company, Great Conversations, on it. The first hour of each class amounts to an informative stand-up routine — Metzger sticks a sanitary pad on her shoulder to show that it won’t slip around — but the second hour is devoted to answering the girls’ questions. Metzger believes that having kids pose questions fosters intimacy and allows parents to hear for themselves what their children’s concerns are. In the first class, when the focus is on the physical changes caused by puberty, Metzger tends to be asked: Why do we have pubic hair? What does it feel like to have a growth spurt? How do I know when I’m getting my period?

As the girls scribbled on their index cards, some used their elbows to block an inquisitive mother’s gaze. (Bolder girls will sometimes go so far as to write things like “This is from Susan in the third row, in the red shirt.”) After intermission, during which Metzger collected the cards into a disorderly pile, she put on a pair of thick red reading glasses and began.

“Can boys stick a tampon in their penis?” she read. “Absolutely not. They can try, but I wouldn’t recommend it.” She flung the card to the floor.

“Do you always get a baby from having sex?” she read. “My husband and I have been married 28 years. We may have had sex over 1,000 times. I am happy to report we do not have 1,000 children. There are ways to show and share your love without having a baby.” Another card flew out of her hand.

“Does having sex hurt?” she read. “When people bring their bodies together, their ear might go into your elbow, but because you have chosen someone you love and trust, you say, ‘Please get your elbow out of my ear.’ And they would say, ‘Of course.’ Do I look like someone who would choose something 1,000 times if it was painful? No, I do not.”

The second class of every course delves into the opposite sex’s puberty, along with reproduction and decision making. Metzger can count on at least one girl asking how you know if you want to have sex with someone. At the class I attended, she got the expected question, then walked briskly to one side of the auditorium and said: “Let’s say it’s 8:12 on a Tuesday night, and you walk by a complete stranger. What would you do?”

“Nothing,” the girls chirped.

“What if it’s 8:12, and you run into Ralph from Jamba Juice, and your family gets a Jamba Juice every Saturday. What would you do?”

“Say hi,” someone yelled. With each question, Metzger moved a few steps toward the other side of the room. “What if it’s your friend whom you haven’t seen since 2:30? What’s your feeling?”

“Happy!”

“What are the consequences? Sleepover! Now what if you spot your grandmother? You give her a big hug, and what’s the consequence? She takes you shopping. But what if I go over to a stranger and shake her hand? What if I give Ralph a huge hug like you did your grandmother?”

The girls snickered. By now, Metzger had reached the other side of the room, her movement reinforcing the notion that different relationships call for different behaviors. “Ohh,” Metzger said with exaggeration. “You’re saying my actions don’t reflect my feelings for these people? If you’re telling me that, then if two people brought their bodies so close that a penis actually went inside a vagina, that’s enormous. If it’s true what you’re telling me, that this seems to be one of the biggest human-being actions, I have to put it together with some of the biggest human-being qualities — trust, respect, love, commitment. That’s why some people say this action belongs only to grown-ups, and that’s why some people say this action belongs only in marriage.”

Boys and girls experience puberty differently. For girls, puberty typically begins at 10 or 11 and lasts five to six years, punctuated by distinct events — breast development and the onset of menstruation. Puberty for boys starts later, around 11 or 12, and lasts longer. Many girls are done with puberty — over, by definition, when growth stops — in their sophomore year of high school. Boys, on the other hand, may still be growing in college, and some secondary sex characteristics, like beard growth, may not show up until they are in their 20s.

The first night of the boys’ course includes a musical interlude, “The Penis Opera,” in which the falsetto of the boys is set off by the bass of their fathers. Preteen boys think saying “penis” is funny, and my son, then 11, guffawed even as he looked around to gauge others’ reactions — perhaps because no one anywhere else ever shouts “penis” at the top of his lungs.

“Maybe you’ve been using the word ‘willy’ or ‘stick’ or ‘twig,’ ” the instructor, Greg Smallidge, a sexuality educator who teaches many of the boys’ classes, told the audience. “We were brought up for generations with people thinking it wasn’t O.K. to name these body parts. That’s why we need ‘The Penis Opera.’ We need to talk about sexuality.”

Yet what that conversation should include is far from settled. In 1913, Chicago’s became the first major school system in the United States to include sexuality as a subject. More than 100 years later, there is still no standardized curriculum. Detailed guidelines, released in 2012 as a resource for school districts, recommend minimum standards for comprehensive K-12 sex ed, but compliance is voluntary. “No state or school district I’m aware of has adopted them in full,” says Danene Sorace, who coordinated the development of the guidelines for Future of Sex Education, a partnership of three nonprofits. As a result, sex ed varies widely in schools. Some places, like New Jersey and Chicago, deliver age-specific lessons starting in kindergarten and continuing all the way through Grade 12. Other places, like Clark County, Nev., home to Las Vegas and the nation’s fifth-largest school district, teach abstinence-based curriculums. Many states have no policies; more than half receive a share of the $50 million that the federal government hands out each year to promote abstinence through community programs.

Great Conversations represents a distinct shift from the usual approach to sex education. Metzger believes that adolescence and puberty should be the purview of children and their parents, not solely that of children and their teachers. “The idea that we are talking to two generations at the same time is at the core of this,” she says.

In a 2012 survey by the National Campaign to Prevent Teen and Unplanned Pregnancy, 87 percent of teenagers said “open, honest” conversations with their parents could help them put off sex and avoid pregnancy. Students who take part in comprehensive sex-ed programs delay having sex for the first time, have less sex and fewer partners and rely more on contraception than their peers. (Conversely, abstinence-only instruction has not succeeded in extending virginity.) “As parents of young children, we are really engaged,” Sorace says. “But sexuality is such a taboo topic in our culture that when it comes to adolescence, we freeze.”

That’s probably why information about sex, whether from parents or schools, is so often delivered in serious, white-coat fashion, its clinical messages heavy with the fear of consequences. To those who advocate abstinence until marriage, attitudes like Metzger’s foster permissiveness. But limiting the conversation to abstinence, Metzger says, “isn’t a full-enough understanding of sexuality.” Because they are voluntary, Great Conversations courses are free to be more frank than school-based sex ed; they can sidestep detractors who think kids shouldn’t be taught about masturbation, for example. “We are not saying you have to learn this,” Metzger says. “People get to choose to come to us.”

Metzger’s greatest challenge might be figuring out how to speak in one voice to families from radically different backgrounds and viewpoints. For the most part, the course, which costs $70, attracts a well-educated, mostly homogeneous demographic. But over the years, Metzger and her business partner, Robert Lehman, who also runs the boys’ curriculum, have tried to appeal to lower-income parents. They found success in Palo Alto, where the class is regularly taught in Spanish. But in Seattle, Metzger says, she has struggled to find a community partner. A deal with the Y.M.C.A. fell through because of the need to simultaneously translate instructors’ rapid-fire delivery into several languages.

Earlier this month, Metzger got an email from a middle-school teacher she knows: Would Great Conversations want to teach a group of disadvantaged students — some homeless, others victims of abuse? Two of her instructors are interested, and Metzger is imagining what shape such a class would take. “It wouldn’t be the same song and dance,” she says.

Metzger’s course might need to evolve in other ways. Lindsey Doe, a clinical sexologist whose YouTube channel, Sexplanations, tackles subjects ranging from kissing to anal sex, attended Great Conversations with her daughter. She was disappointed that the focus was limited to either boys or girls. Where would a transgender or an intersex child fit in? “I loved the curriculum so much that I wanted it to be perfect, and that was the piece that would have completed my experience,” Doe says.

Metzger is open to the idea. Finding the right words to include adoptive families was tricky when she started teaching the course; now, it’s how to deal with sexual identity. “There was a titanic shift five years ago when the audience began demanding a more open conversation around homosexuality and transgender experiences,” she says. “We’re always trying to balance the readiness of the room, and we may be running a bit behind.”

In November, my 10-year-old, Shira, and I attended For Girls Only. There was an undercurrent of nervous tension as we waited for the class to start. Mothers looked stressed, daughters embarrassed. Shira hadn’t wanted to come. “I don’t want to learn about puberty,” she pouted. “I don’t even like the word.” But as the girls looked around, some of them spying friends, they seemed emboldened: Maybe theirs weren’t the only parents to drag them to a talk about penises and vaginas.

And then Metzger won them over. At one point, she handed out a diagram of a woman’s reproductive organs and challenged the girls to go home, stand naked in front of a mirror and superimpose the image over their abdomens to get a sense of where things were in their bodies. When Shira’s drawing fell to the floor, she gave me an impish grin and asked, “Mom, could you pick up my uterus?”

Later still, she leaned forward, intrigued, when the talk turned to how to insert a tampon; I’d never explained that to her. “Some people worry they’ll put it in too far,” Metzger was saying. “What if you’re in social studies and it comes out your ear?” She pantomimed stumbling across the room and pulling a tampon out of her ear; lots of laughter followed her. “That — ” Metzger paused dramatically — “cannot happen.”

A month later, on a drizzly December Monday, I met with Leah Likin, now 14. She has long, curly hair that fades from brown to blond, and she twirled one lock around and around as she talked. I asked her why she was dead-set against going to Metzger’s class three years earlier. She struggled to explain herself. At last she said, with a blush that highlighted her freckles: “I guess I didn’t want to grow up. I was happy with the way things were. I am realizing now that the class was superhelpful. Julie sends you away with this greater message that we are all in this together, that you’re fine,” she said, referring to Metzger. “That’s what my mom always says: You are just right the way you are.”