There’s a debate going on on Feministing about single-sex schools (and it’s even flame-free!), following a recent study that shows that girls in the UK who go to single-sex schools later make more money in life than girls who go to coed schools. So far the situation seems to be that prestige is an alternative explanation for that.
Actually, Echidne’s post about problems in social science reporting provides an entire set of problems for the study. For example, the study looked at people born in 1958, and mentions that girls who went to single-sex schools took more science and math courses; now, when even in math and science the high-school gender gap has closed or even reversed, it may no longer be applicable.
Even if you accept that British girls born in 1958 were causally better off in single-sex schools, in the sense that if in 1969 parents had sent their daughter to a single-sex grammar school she’d have made more money later in life, it’s hard to conclude that encouraging single-sex schools is a solution.
For a start, separate is never equal. A few months ago, NOW President Kim Gandy pointed out how gender segregation was effectively a back door for underfunding job training for women and girls’ sports programs.
There’s established research showing that occupations that become female-dominated also become less valued over time. So it makes sense that female-dominated education will be considered inferior to male-dominated education, even if both are considered superior to co-education. At a minimum, to show that gender segregation helps anything, one needs to show that the gender gap between single-sex alums is smaller than the gender gap between coed alums, controlling for occupation.
Finally, there’s the effect of gender segregation on men. I don’t know about any study showing that men in gender -segregated environments are likelier to be sexist, but I know enough data points to suspect a trend. In that case, if it’s causal, then co-education is necessary to maintain political support for feminism.
Abstract Nonsense 
co-education is necessary to maintain political support for feminism.
Even if this is true, I don’t think this is going to be a very successful argument — you’re essentially telling parents to sacrifice their daughters for the greater good. And the people least likely to care about the greater good are parents making decisions about their kids’ educations.
That’s only how I framed it here, because the Feministing discussion was mostly about the broader feminist implication. There’s a more individualistic way of putting it: sending your daughter to a single-sex school will make it more likely that her male coworkers will discriminate against her because they grew up in a homosocial environment.
Though now that I think about it, you could also argue that there’s a vicious circle here: any small aberration in favor of single-sex schools will be used in recruitment materials, which will make parents who are especially concerned over their children’s future success send them to single-sex schools, which will populate single-sex schools with more driven students, which will make single-sex schools look even better – all this without improving a single student’s education or drive to succeed.