Molio
Interesting anecdotes on girls schools, but factually incorrect (and logically so). No doubt there will be plenty of people saying what a terrible time they had at a girls school recounting stories of the school "bitches" - for which you have my sympathy, but you're extrapolating your experience without foundation.
Both the NSPCC and the DoE have commissioned surveys into the overall issue of bullying and discovered, incidentally, that there is less bullying in girls schools. Further surveys confirm the obvious that, of the nearly 1 in 6 (some say 1 in 3) girls that suffer some form of sexual harassment or abuse between the ages of 11 and 16 at school virtually none of it occurs at girls schools. When you don't have knuckle dragging teenage testosterone monsters around less of this stuff happens.
On top of this, the idea that girls somehow stop huddling together, bitching and competing over appearance or indeed male attention because some boys are playing football or looking at bikinis on their phones is, in my humble opinion, utter rubbish. In fact, the presence of boys is more likely to encourage this behaviour.
In fact, every girls school I've seen (open days, discussions with parents/students) compared to many co-eds makes it clear that the pastoral systems are most often much better at coping with PSHE issues for girls - the teacher/girl ratio alone makes that possible, let alone the ability to focus on issues (many of which are not relevant for boys). For instance, remember that eating disorder incidence is to high amongst adolescent girls (and boys) across the entire UK - most of which is in a co-ed setting. There is no evidence that it is any higher in single sex settings at all (prove me wrong please) and there is plenty of evidence that girls schools are better set up to identify and help.
This is born out by the work done on comparing subject choices in single sex and co-ed settings. It is not surprising that in single sex settings boys and girls don't follow "gender stereotype" subjects. Boys will choose arts and languages more often and girls will choose maths, physics and the sciences more often. Put them in a co-ed setting and all the stereotypes are expressed in subject choices - girls in particular shy away from maths and physics - an issue for wider society well discussed.
Take all this evidence, which seems very clear and obvious to me, and add in the icing from the Centre of Longitudinal Research that shows women earning more money following single sex education and marrying just as often (the indicator used for relationship forming).
Oh - and did I forget that girls also perform better academically across the spectrum of schools - private, faith, state - they always perform better - buts that so blindingly clear from the dreaded "league tables" it's hardly worth debate.