Single-sex academies like these two Dallas schools not only benefit the students fortunate enough to attend but also are a part of the solution to the growing boy gap in education and the persistent girl gap in math and science. Today millions of American boys are languishing academically. Boys in all ethnic groups and social classes are far less likely than their sisters to feel connected to school, to earn good grades, or to attend college. Girls, by comparison, are thriving academically, but are still far less likely than boys to enter fields in science and technology. Boys and girls, taken as groups, have different interests, propensities, and needs. These academies can provide important lessons on how to educate our children more effectively.

What sensible person would call these programs and others like them morally and legally suspect? And yet the American Civil Liberties Union says they are. It has launched a major campaign called “Teach Kids, Not Stereotypes” to discredit and terminate gender-specific programs in American schools. For the ACLU, organizing by sex is analogous to organizing by race. Lenora Lapidus, Director of the Women's Rights Project of the ACLU, seems baffled so few see the connection. “It is, in fact, quite startling,” says Lapidus, “that many people who would never consider segregation based on race or religion in a public school would accept sex segregation.” This campaign is unlikely to advance the cause of social justice. But its potential for mischief is abundant.

No one is suggesting that any student be forced into a single-sex program. Coeducation is an American tradition, and that will never change. But some parents and students prefer girls' and boys' schools, so why deny them that choice? What could be wrong with a voluntary program that seems to be doing so much good? Plenty, says the ACLU, and it claims to have research to prove it: a 2011 critique of single-sex education published in the prestigious journal Science.

The Science article is a two-page summary of the state of the literature on single-sex education. That could be useful, except that it was written by eight professors who belong to an advocacy group that opposes single-sex education. The article attempts to persuade readers of two propositions: 1) There is no well-designed research that proves that single-sex education improves academic achievement, and 2) there is good evidence that sex segregation increases gender stereotyping and “legitimizes institutional sexism.”

On the first point, there are in fact many studies that demonstrate the value of single-sex education. A recent article in Demography showed that both men and women benefit significantly from the single-sex model. A 2008 German study found that young women in female-only physics classes developed more confidence and interest in the subject than their counterparts in coed classes. When a group of researchers at Stetson University compared single-sex and coed classes in a Florida elementary school, they documented large gains for both boys and girls in the single-sex classes—but especially boys. Over the four years of the study, 55 percent of boys in coed classes scored proficient on the Florida Comprehensive Assessment Test, compared with 85 percent of boys in the all-boys classes.