A leading girls' school head teacher has said it makes her "mad" when people dismiss single-sex education.
Research findings present a mixed picture
Cynthia Hall, head of an independent girls' school in Oxfordshire, said she believed separate education benefited most girls "enormously".
She told the Girls' Schools Association (GSA) conference that girls were given a positive image of their capabilities.
But her comments were dismissed as "special pleading" by the leader of the National Association of Head Teachers.
Mrs Hall is headmistress of the School of St Helen and St Katharine in Abingdon and current president of the GSA, which represents more than 200 independent, single-sex schools in the UK.
She told its conference: "It makes me mad when I hear heads of co-ed schools dismiss single-sex education with the comment that the co-ed classroom is natural, as if being natural is all the justification one ever needed for anything.
"I believe that most girls benefit enormously from being in a single-sex environment during their school years."
A survey published by the association found that 90% more of its schools' girls chose physics or chemistry at A-level than in all schools nationally.
Mrs Hall said girls' education could suffer when they were taught alongside boys.
"In the teenage years, when girls are finding out who they are, the ability to camouflage in order to fit into a given environment is a highly perilous quality for girls," she said.
"It particularly makes them vulnerable to verdicts of others about their own incompetence.
"These years for girls coincide with the equally important years for boys in which they are testing out their strength, voicing claims they cannot yet deliver, seeing how much they can dominate the world around them."
Breaking the rules
She gave as an example the gender differences she had seen in debating competitions.
Boys would be aggressive, break the rules and say things they did not believe in order to win.
Girls were very concerned to stick to the rules and got "very hot under the collar" about boys who won the debate then said "I didn't believe a word of that".
But the general secretary of the National Association of Head Teachers, David Hart, said he totally disagreed with Mrs Hall's "rather old-fashioned" views.
"It sounds to me very much like special pleading," he said.
He could understand her wish "to run a propaganda campaign for single-sex education" but it would have been more sensible if it had been based upon valid evidence.
Research on the subject has presented mixed findings.
Girls' schools had good exam results because they had high-achieving pupils, not because they were single-sex, according to a 1999 study by London University's Institute of Education.
Ability as well as social class and the history and traditions of schools had a greater impact on the results girls achieved, it suggested.
A 2002 study by the National Foundation for Educational Research suggested girls in single-sex comprehensives achieved better results than girls in mixed schools, especially in GCSE science.
It also suggested separate schooling particularly benefited those at the lower end of the ability range.
The same was also true for boys - though overall there were no differences between the performance of boys in single-sex and mixed comprehensives.
It found girls' schools "do at least help counter the traditional stereotyping".
Girls educated separately were more likely to study resistant materials, less likely to take food technology, and more likely to take separate sciences.
Another 2002 study by Cambridge University's faculty of education analysed results from a mixed comprehensive which taught children separately for some of the time.
Its GCSE results suggested that both girls and boys achieved very much better than all boys and girls nationally - the boys especially so.
"In the classroom the girls were found to be extremely dominant in mixed sex groupings, setting the pace, potentially at the expense of some boys' learning," the authors said.
They regarded this as a strength rather than a weakness, arguing that the need to focus on girls' achievements and aspirations was as important as the need to find strategies to improve boys' commitment and achievement.