I think they can fight blatant sexism pretty well, but often fall victim themselves to perpetuating more subtle sexism - for example my girls' boarding school made great efforts to say we could do anything, but then failed to offer opportunities such as design/tech for GCSE or decent computer tuition, and made no efforts to counter the idea that maths was for geeks and the Chinese. On the plus side, it did have excellent science teachers, including the female head of Physics who explained that when she was at uni in London 30 years earlier, she was the only woman among 300 men and never had to buy her own lunch.
Times have changed, she said. Now you would be one of around 10 women out of 300 students. And you'll have to buy your own lunch.
And while careers guidance emphasised we could do anything, the line was always "how are you going to support yourself doing X" - not once was it ever suggested one might want to be able to afford to support a family. Talks were on medicine, law, and various arty careers - engineering, computing, government etc weren't covered.
Only having alumnae do Speech Day was also restricting in that we only had two very mildly famous ones, one of whom was only for whom she married.
Thing is, single-sex schools and their staff and pupils are still part of society. In some ways there was more obsession with looks, weight, and getting a boyfriend than there might have been if the lads from [nearby schools] had been hanging around all the time instead of only when they'd scrubbed up for a Saturday night dance.