“She’s following her passions!”

“At least she’s not sleeping with folks for money.”

Wylie regretted to inform her girls that lust is one of the seven deadly sins, which prompted the thoroughly modern question: “But how is lust bad?”

Wylie says she believes she is a better teacher, and her students are better students, because they’re in a desexualized  or at least less-sexualized  environment. “Sure,” she says, “when they take pictures, they often present their backsides first. But I think I’m giving girls a better education than I could have if there were guys in the room. I’m freer. I’m more able to be bold in my statements. When I teach poetry and I talk about the sex in poetry I don’t need to be worried about the boy in the room who is going to chuckle over the thing he did with the girl last week and embarrass her. Which happened more than once in my last coed environment.”

Nearly everyone at T.Y.W.L.S. acknowledges that often parents’ most pressing concern when enrolling their 11-year-old daughters is sheltering those girls from sexualized classrooms and sexualized streets. “Harlem’s a very intense environment,” says Drew Higginbotham, T.Y.W.L.S.’s assistant principal, who lives in the neighborhood. “You’re constantly needing to prove yourself physically, to prove yourself sexually. Parents, when they come to our school, they sort of exhale deeply. You can hear them thinking to themselves, I can see my daughter here and she’s going to be O.K. for six hours a day.” Sax is not above or beyond this kind of thinking, either. In fact, after a nearly-two-hour conversation filled with scientific jargon and brains, he told me, perhaps wishfully, that really the most important reason to send a child to a single-sex high school was that those kids still go on dates. “Boys at boys’ schools like Old Farms in Connecticut, or Saint Albans in Washington, D. C., will call up girls at Miss Porter’s in Connecticut, at Stone Ridge in Maryland, and they will ask the girl out, and the boy will drive to the girl’s house to pick her up and meet her parents. You tell kids at a coed school to do this, and they’ll fall on the floor laughing. But the culture of dating is much healthier than the culture of the hookup, in which the primary form of sexual intimacy is a girl on her knees servicing a boy.”

In the past few years Tisch’s Young Women’s Leadership Foundation has opened schools in the Bronx and Queens, as well helping start ones in Chicago, Philadelphia, Dallas and Austin. Tisch wants to be careful about not overextending her network  “we don’t want to become Mrs. Fields or Benetton”  but she says she also feels an obligation from her success. Last year, 2,100 students applied for the three open ninth-grade spots in the Harlem school. Many other schools make inquiries about how they might replicate T.W.Y.L.S.’s success. This coming year, for the first time, Tisch plans on holding her own conference on single-sex public education. Though she’s meticulously circumspect about not disparaging Sax, her actions suggest that she is aware that if she doesn’t engage with the many districts interested in starting up single-sex programs, there’s a chance that Sax will run away with the movement.

Education scholarship has contributed surprisingly little to the debate over single-sex public education. In 2005, the United States Department of Education, along with the American Institute for Research, tried to weigh in, publishing a meta-analysis comparing single-sex and coed schooling. The authors started out with 2,221 citations on the subject that they then whittled down to 40 usable studies. Yet even those 40 studies did not yield strong results: 41 percent favored single-sex schools, 45 percent found no positive or negative effects for either single-sex or coed schools, 6 percent were mixed (meaning they found positive results for one gender but not the other) and 8 percent favored coed schools. This meta-analysis is part of a larger project by the Department of Education being led by Cornelius Riordan, a Providence College professor. He explained to me that such muddled findings are the norm for education research on school effects. School-effects studies try to answer questions like whether large schools are better than small schools or whether charter schools are better than public schools. The effects are always small. So many variables are at play in a school: quality of teachers, quality of the principal, quality of the infrastructure, involvement of families, financing, curriculum  the list is nearly endless. Riordan says, “You’re never going to be able to compare two types of schools and say, ‘The data very strongly suggests that schools that look like a are better than schools that look like b.’ ”

That certainly appears to be the case for single-sex schools. The data do not suggest that they’re clearly better for all kids. Nor do they suggest that they’re worse. The most concrete findings from the research on single-sex schools come from studies of Catholic schools, which have a long history of single-sex education, and suggest that while single-sex schools may not have much of an impact on the educational achievement of white, middle-class boys, they do measurably benefit poor and minority students. According to Riordan, disadvantaged students at single-sex schools have higher scores on standardized math, reading, science and civics tests than their counterparts in coed schools. There are two prevailing theories to explain this: one is that single-sex schools are indeed better at providing kids with a positive sense of themselves as students, to compete with the antiacademic influences of youth culture; the other is that in order to end up in a single-sex classroom, you need to have a parent who has made what educators call “a pro-academic choice.” You need a parent who at least cares enough to read the notices sent home and go through the process of making a choice  any choice.

As T.Y.W.L.S. let out on a Friday in January and the girls spilled onto 106th Street, one such parent, a man in saggy jeans and a black parka, walked up the sidewalk clutching his daughter’s dog-eared report card and hoping to secure her a spot for next year. “This where the school at?” he asked a security guard. The engagement of parents like this may be a major part of the success of single-sex public education. These schools are popular with many parents, who are happy to have an option that has long been available in private and parochial schools. And they are also attractive to teachers and administrators, who are offered a relatively easy and inexpensive way to try to improve some of the intractable problems in public education, especially for disadvantaged students.

But schools, inevitably, present many curriculums, some overt and some subtle; and critics argue that with Sax’s model comes a lesson that our gender differences are primary, and this message is at odds with one of the most foundational principles of America’s public schools. Given the myriad ways in which our schools are failing, it may be hard to remember that public schools were intended not only to instruct children in reading and math but also to teach them commonality, tolerance and what it means to be American. “When you segregate, by any means, you lose some of that,” says Richard Kahlenberg, a senior fellow at the Century Foundation. “Even if one could prove that sending a kid off to his or her own school based on religion or race or ethnicity or gender did a little bit better job of raising the academic skills for workers in the economy, there’s also the issue of trying to create tolerant citizens in a democracy.”