America’s flirtation with single-sex schools is threatening to fizzle out after the release of a sceptical new survey on their benefits for girls.

Six years ago, the American Association for University Women said that the tendency for girls to be shut out in co-educational classes helped explain their weaker results, particularly in science and maths.

The report inspired experiments in single-sex schools and classes in states from California to New York. But the organisation, in an apparent volte-face, now insists there is “no evidence in general” that single-sex schools are better for girls.

“What the research shows is that separating by sex is not the solution to gender inequity in education,” said Maggie Ford, president of the AAUW foundation that issued the report, Separated by Sex: A Critical Look at Single-Sex Education for Girls. “When elements of a good education are present, girls and boys succeed,” she added.

Only last year, the first of several single-sex state schools opened in California, in a $5 million (pound;3m) experiment. The benefits touted are that they may reduce gang violence for boys while enabling both sexes to concentrate without distractions.

US studies have consistently suggested that while girls equal or outperform boys until they reach adolescence, they then fall behind, particularly in maths and science. Girls of this age are widely said to be insecure and lacking confidence.

But it was the AAUW’s work that was the first to blame classroom sexism in the co-educational environment. All it is saying now, officials insist, is that single-sex education is not some magic solution, and can sometimes reinforce gender stereotypes.

The AAUW report collated work from Catholic and private schools in the US, as well as several surveys around the world. In Northern Ireland, for example, researchers concluded that single-sex schools improved girls’ self-esteem. However, in New Zealand, it was found that the behaviour of teachers was far more important than whether their pupils were segregated by gender.

The evidence, the report noted, is at the least confused. The move to single sex classes was often matched by extra money which also brought smaller classes, more committed teachers, and a tighter curriculum.

“The key finding of this research is that any school, single-sex or co-ed, that sets high standards and has a rigorous academic programme will give young women the skills they need to succeed,” said education secretary