A leading headmistress has cautioned against the trend for all-girls boarding schools to go coeducational.

Frances King, head of Heathfield St Mary’s School in Ascot, Berkshire, said that mixed boarding schools can damage girls during puberty by lowering their self-esteem and denting their academic confidence.

The trend for educating boys and girls together had been accepted uncritically for far too long and needed to be questioned before the pressure for all-girls boarding schools to go coeducational became too great, she said.

“Puberty is a very difficult time and the self-confidence of girls can be extremely fragile,” she told The Times. “Is it right to be throwing into the melting pot the complication of sexual dynamics? Why add to the emotional cocktail by subjecting them to the intensity of a mixed boarding environment?”

The annual census from the Independent Schools Council, due to be published this week, is expected to show a drop in the number of pupils attending girls-only boarding schools.

Advocates of single-sex schooling have long argued that children, particularly girls, achieve more academically when they are taught separately, taking as evidence the fact that girls’ schools consistently top school league tables.

But a research review last year by the University of Buck-ingham found no evidence to support this.

Ms King said that it was becoming increasingly difficult to find single-sex boarding schools as more and more boys’ boarding schools decided to go coeducational to bolster their numbers and improve their position in the academic league.

Girls attending mixed day schools did not face the same pressures as boarders because they could go home every evening and find support from adults, she said.

“But at a boarding school, they have to rely more on peer support, which can become destabilised, for example if you have to fear that your best friend might fancy your boyfriend,” she added.

Olivia Paterson and Alice Orr-Ewing, both 17 and pupils at Heathfield St Mary’s, agreed that girls at single-sex schools had plenty of time to socialise with boys at formal events or joint school plays with boys’ schools. Louisa Matheson, also 17, said: “I just want to be able to focus on my friendships and my studies.”

But Mike Buchanan, head of Ashford School, Kent, an all-girls boarding school that will open its doors to boy boarders in September, said it was important to expose children to different learning styles and ap-proaches to life. “Girls and boys can learn a lot from each other,” he said.

Anthony Seldon, master of Wellington College, previously an all-boys school that admitted girl boarders at all levels last September, said: “We need to bring [pupils] up in as broad a way as possible with the other gender and, ideally, with people from other races, religions and social backgrounds.”

Where all the girls have gone

— Thirty years ago there were nearly 24,000 girls in the leading single-sex boarding schools and only 2,000 at top coeducational institutions

—There are now 11,000 girls in the leading mixed boarding schools, nearly 500 more than attending the elite schools in the Girls School Association

— More than 130 independent single-sex schools for boys and girls have merged, gone coeducational or closed

—The number of single-sex state schools has declined from 2,500 in the 1960s to about 400 today

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