Quite a bit of discussion re boarding schools is a bit out of date. The modern world cannot be kept out of schools in the way it once might have been. This can cause significant pastoral problems when staff are in loco parentis, but I don't believe there is the isolation from the opposite sex or the world that is suggested. For a start, few schools now have sixth formers who remain cloistered at weekends.
When they attended a highly competitive co-ed day school, my daughters used to get up 20 mins early to put make up on. When we went overseas they went to a girls' boarding school. Now they don't apply makeup for school. Life is much easier when you don't have panda eyes by lunchtime and after PE.
Today's article, linked below by Rachel Johnson, about end of term at Marlborough suggests time spent on looks is not confined to single sex girls' schools. I would say there is far less fuss at my daughters' equivalent event - perhaps because there are fewer boys?
www.telegraph.co.uk/family/9373117/Girls-today-are-so-glamorous-but-theres-a-price-to-be-paid.html
I have taught extensively in co-ed schools and in two boys' schools, one an outstanding comp (ex grammar) and one which was socially very selective but with a huge ability range. (day school) I didn't find the boys sexist except where they were simply being adolescent and learning how to behave. I generally found them interested to learn and keen to discuss and ask questions about relationships and feamle perspectives, many of which they would not have asked in front of girls. There is less inclination to show off if no girls are around and discussions were frank.
I don't have strong feelings either way - I think it is the quality of the school that is important, not the single or mixed sex aspect, though I understand anyone who feels the set up is unnatural and would not want single sex education.
However, the idea that there is more anorexia etc in girls' schools is not especially true in my experience, once you have taken into account the competitive nature of families who send their children to high-achieving schools. There is also a certain amount of affluent neglect in some families with children who board and other family issues (quarrelling parents etc) - money changes aspects of children's problems, but problems there are.
I would say from my observations that it is family issues and pressures that most frequently lead to MH problems (anorexia/self-harm etc) and the schools have to deal with the fallout sometimes with parental support but not always. I find the girls themselves generally make fine and stalwart confidantes and supporters of each other.