I think that part of the issue with the single sex vs mixed question is that a lot of single sex girls' schools are (a) private, and (b) academically quite intense.
I went to a single sex girls' comprehensive, which was not particularly academic at all, and never saw any of the bitchiness or 'hothouse' effect. No anorexics that I knew of, quite a few teenage pregnancies - but I think that was also true of the nearest equivalent mixed school on the other side of the big estate that both schools served.
We hung out at break and lunchtimes with the boys from the boys comp across the road, so no particular socialising issue, and were mixed for the sixth form (not many in either school stayed on past 16 though, maybe 10% or less).
I hope that it isn't an issue now, but certainly compared to my friend who went to a mixed school, there was much more chance as a girl to take maths, physics etc without getting grief at my school. Equally, though, the disadvantage was that I didn't have the chance to take Further maths A level, as it wasn't offered - I think it was at the mixed schools - but then only boys really took it, so you couldn't win . . .