Unpublished research by OFSTED reveals that girls' schools outperform boys' not only on standards but also in terms of ethos, efficiency and quality of education.
Girls' schools have long dominated the GCSE exam league tables, holding nine out of the top ten places since 1994.
Critics have argued that their success was due to selection rather than gender - that single-sex schools recruited from more favourable socio-economic backgrounds and benefited from long-established academic traditions.
OFSTED's research does show that boys' schools often have more disadvantaged catchment areas. However, when the researchers compared more than 800 schools - boys', girls' and mixed comprehensives - the girls' schools came out on top on attitudes to learning, progress at key stages 3 and 4, attendance and behaviour.
The findings were revealed by Christine Agambar, OFSTED's head of research, at the Association of Maintained Girls' Schools conference in Oxford last week.
And they are good news for the association, which celebrates its 10th anniversary next year.
Joan Senior, head of Brentford school for girls in London, said: "People's view of girls' schools is a grammar in a leafy suburb and that perception is where part of the problem lies. Girls' schools have their share of the problems too."
Last year 46 per cent of her pupils were on free school meals and 38 per cent had English as a second language. "We are near Heathrow. Look at any of the trouble-spots in the world and within three weeks to a month we will have children from those areas in our school," she said.
The association is to commission more research into the effect of girls' schools.
Mrs Senior explained: "We need to know how those girls who will never get five A-Cs are performing and whether they are performing better because they are in a girls' school."
But while the sector is celebrating success, Gillian Plummer, an educational consultant, said serious questions still needed to be asked about the achievements of working-class girls. Last year they accounted for only 1.6 per cent of all students in higher education.
"Educational success helps you escape your parents' position but it has a huge emotional cost," she said. "You are told you're not speaking properly when all your family speaks like that, that you have bad manners and eat badly.
"If you fit in one place you feel alienated in another."
A review of earlier academic research on girls in single-sex schools for the association found there was no conclusive evidence that single-sex schooling was better than co-educational schooling.
The review, by Jannette Elwood and Caroline Gipps, from the Institute of Education at London University, said there was no general rule that one type of school was better than another.
GIRLS' SCHOOLS THE STATISTICS
There are 231 local authority maintained girls' schools - 226 secondary, one primary and four specials - and 340 girls' public schools.
Of the 226 maintained secondary schools, 149 are comprehensives, nine are secondary moderns and 60 are grammars. The Department for Education and Employment says there are eight "other".
At least 70 maintained girls' schools have pupils on free school meals. This includes selective schools. Eighteen maintained girls' schools have more than 30 per cent of their pupils on free school meals and at one comprehensive 75 per cent qualify.