Originally, separating classes by sex was a measure for the benefit of girls:
The reformers’ talk was all about “How Schools Shortchange Girls” (the title of a much-discussed report from the American Association of University Women) or were “Failing at Fairness” (the title of a 1994 book) for young women. Girls were said to be hitting a self-esteem barrier in junior high, when they would droop like Ophelias in need of reviving. But that did not turn out to be the long-term problem. Increasingly, girls have been besting boys’ performance across the board: G.P.A.s, school leadership, graduation rates, college attendance. Now segregation by gender is more often an intervention for boys—a switch in target that makes you wonder a little about the reliability of the justifications for the policy.Most of the secondary schools (or, rather, high schools) in our area are single-sex. As with most of these things, the data relating to their performance demands a bit of reasonable doubt. One has to consider whether it is the single-sex policy itself that is causing improvement in academic performance, or whether other factors are more influential. More prestigious institutions typically remain tethered to their single-sex origins — so it's understandable that the schools that appear desirable to parents operate under a 'boys only' or 'girls only' rule. The fact that only one gender attends the school tends to be immaterial.
At a classroom level, there's a little more rationale behind the idea. But, as the Margaret Talbot alludes to, the presumptions on which the policy is based are mostly silly and generally false. (Apparently girls like reading; boys don't. Also, did you know that boys like running, and girls don't? Furthermore, I bet you had no idea that it's actually impossible for girls to outperform boys in mathematics!) There's a kind of absurd pseudoscience to prop up these long-held and well-worn stereotypes, but it rarely holds water as justification for segregation in the classroom.
This is not, as some people have suggested, a question of fairness. It's a matter of which system ensures the best schooling for students, both male and female. There's something off-putting about the double-standard when it comes to single-sex classrooms — not to mention the stupidity of its justifications.