I think that, once again, it's to do with personality.
Fwiw, I went to an all girls GS (so another factor in there, again!) but I believe that whilst it was fine, 11-16, it was waaay off the mark 16-18. It was rather ridiculous, the rarefied, blue-stockinged air, the still-in-full-uniform thing etc etc. The girls who thrived were, frankly, those who were a bit scared of the opposite gender- many applied to womens colleges as well.
There wasn't much bullying at all, I recall but possibly this was because a) we were, to put it bluntly, perhaps cleverer, more articulate and often with stronger personalities therefore were less likely to succumb to bullying, and b) being selective, they could (and occasionally did!)- deselect.
Another interesting thing I've heard is how in favour of a mixed comp many of my female friends at work are- however, they are, to a woman, better looking than average. I think this is a serious point, ie a clever, pretty girl would possibly find a mixed comp an easier ride than an average looking girl of any academic level.
Finally, I've read that parents of boys want their DSs in mixed schools, parents of girls want their DDs in single sex schools!