i think it depends on the child. if a girl only has sisters, or is an only child, and her father is fairly authoritarian, then sending her to an all girls' school could be a problem because she won't have any experience of boys/men other than her father, and then to launch into 'life' aged 18, meeting boys as friends/work colleagues for the first time, its likely she might find herself behaving with them exactly as she behaves with her father, i.e., not speaking up for herself and/or saying yes the whole time. Which is a problem.
I think it's so much better that children go to a co-ed school where they see all the little tentative 'relationships' starting - they 'go out' with each other for about 3 days, then 'split up' but they never actually go on a date or do anything physcal, then in about Year 9 or 10, maybe they 'go out' for a couple of weeks and there's a kiss on the cheek involved and slowly, slowly they learn to get used to world of relationships and are not dropped in at the deep end at 18 with no experience whatsoever, where other people have already learnt to say no, or set boundaries.
Equally, if all your debating classes are girls only, how are you going to cope at university when faced with young men in your class? I'm not convinced that avoiding boys until you're an adult actually makes you more able to compete with them at all.
Co-ed for mine all the way.