This week’s newspaper photographs of smiling teenagers excitedly showing off their grades are a vivid reminder that this is A-level results week. You may, in passing, have remarked on how many of these ecstatic students were female. If you assumed this was a case of media sexism, however, you’d be mistaken (partly). For yet again, when it comes to exam glory, independent all-girls’ schools are out in front.

Every year, single-sex schools dominate the league tables for A-levels and GCSEs. But if boys and girls do so much better academically when kept apart, why is segregation of the sexes not more prevalent?