It is one of the defining characteristics of the Victorian schoolyard: the twin doors, one labelled "boys", the other "girls". But the idea of separating girls and boys is widely considered as relevant to modern education as inkwells, chalk and spinster schoolmarms.
Until now. Mike Younger, head of the education faculty at Cambridge University, last week told a conference of headteachers and academics that he had witnessed a definite resurgence in single-sex teaching in mixed comprehensives in recent years. And the inevitable newspaper headlines followed.
"It can have a real impact on achievement and motivation levels," he said. "It's about trying to persuade more girls to take science and maths in the sixth form, and trying to persuade boys to achieve in English and languages."
Mr Younger estimates that 5-10 per cent of comprehensives are experimenting with single-sex teaching. But it is rarely applied universally: schools tend to separate pupils only for key subjects, such as English, maths, science and languages.
"They've chosen to go to a mixed school," says Rob Jeckells, deputy head of Kings' School in Winchester, which runs single-sex classes for GCSE pupils. "So we don't want to push it too far. If we extended it across all subject areas, it would lose that specialness."
Mr Jeckells believes that the slightly erratic nature of these classes is part of their appeal. His school introduces them only when staff believe that pupils in a particular year group would benefit. "It's perceived as unusual and special," he said. "The pupils recognise it's unusual, and they want to concentrate.
"When you put lads together, their natural competitiveness takes over. They are not attempting to show off in front of the girls. They want to do well. They start believing they can succeed."
Empirically, however, the impact of sexual segregation is still unknown. Stephen Gorard, professor of education at Birmingham University, has researched the effects of single-sex schooling on pupils, and insists that results are inconclusive.
"What are you measuring?" he says. "If you are looking at paper-and-pencil tests, there is no evidence that girls or boys educated separately produce better scores."
He concedes that the advantages of such classes might not be purely academic: segregation may well enhance pupils' confidence and self-esteem. But pupils' attainment is measured far more easily and consistently than their emotional development.
"So it seems anathema that we segregate on any basis whatsoever," he said. "It's just reinforcing stereotypes."
Nonetheless, most schools choose to introduce single-sex classes precisely in order to minimise gender stereotyping. And with the increased focus on boys' underachievement, staff in mixed classes often forget that many girls still lack confidence in maths and science.
At Kings' School, single-sex classes began as a way of tackling underachievement among boys at English GCSE. "You can tailor the way you teach to the characters in each class," Mr Jeckells said. "We start with boy-friendly content, and then widen out. Eventually, we put texts in front of them that they previously wouldn't have looked at.
"And boys on their own are more willing to share ideas about poetry, Shakespeare, drama. They're reluctant to talk about those sorts of things in front of girls - they worry about their credibility, being perceived as people who aren't cool."
Since single-sex classes were introduced, increased numbers of Kings' boys have gone on to sixth-form college and university. But Mr Younger insists that schools should examine their motives carefully before corralling boys and girls into separate rooms.
"It's been seen as a way to narrow the gap between boys and girls," he said. "But we don't think it will necessarily do that. And there's an issue in some schools with invisible girls, underachieving because the focus has been on the boys."
For single-sex classes to make any difference at all to pupils' attitudes, Mr Younger insists they must be implemented properly. This takes time: ideally, schools would run them for two or three years before determining whether problems outweigh advantages. And staff have to be actively in favour of the idea rather than merely passively allowing it to happen. This is particularly true of senior management.
"You need a committed group of staff willing to run with it, to explore challenges and difficulties," he said. "The leadership needs to sell the idea to the whole school, to pupils and parents. And teachers need to be willing to take risks, to admit problems. Otherwise it's not going to be very effective."
And, Mr Jeckells adds, pupils need to be convinced of the benefit of the classes as well. "Pupils need to be clear from the start that we don't want to punish them in any way," he said. "The boys were concerned that we were putting together all the boys whose behaviour was challenging, so that the girls could do even better. We had to make it very clear that we were targeting them because we wanted them to achieve."
Professor Gorard, however, questions whether success is ever guaranteed. "We want to encourage people to innovate and try things out," he said. "But there has to be rigorous evaluation, not just 'it makes me feel good'. That is not the stuff government policy should be made from."
The very word "segregation", he believes, should serve as a warning sign to schools. "If you substituted 'sex' with anything else, people would be appalled," he said. "We would not allow segregation by language or ethnicity.
"There have been attempts to teach black boys separately, and people were uncomfortable with it. The aim with special-needs pupils is not to have them taught separately. So why do we segregate by sex?"