I can see fors and againsts for both. I went to a single sex convent from age 11, and whilst it was quite a while ago, in retrospect, there was a feeling of developing along separate lines, and that the male of the species, apart from one's father or priests, were not to be trusted, insomuch as they would always be trying to wear, us girls down with a view to stealing our virtue. That notion was propagated by the nuns that tought us, who I now realise in fhe fullness of time were definitely on the far end of the batshit spectrum and really shouldn't have been around children.
However, having said that I know two girls quite well, who went to private single sex schools in more recent times. One is a son's girlfriend of five years or so and the other is a daughter of one of my closest friends. Both very nice people, but it does stike me that they don't have the vast array of friends of both sexes that my sons have, many female friends retained from their days at the local comprehensive. Both these girls mentioned appear to have smaller social circles. The mixing of sexes can be a positive insomuch as it did appear to me as a parent of boys going through a mixed sex school they were far more relaxed around the opposite sex than I was at their age.
The downsides, undoubtedly, sexual harrassment, not to mention the relatively new phemenon of sexting and the humiliation that inevitably ensues. Possibly some girls don't reach their full potential around boys, maybe some might feel they boys are an intimidating presence. I can only speak for myself, but at such a difficult age, the introduction of gender neutral toilets would have been embarrassing in the extreme. I can imagine some girls avoid going in such situations, which isn't good.
On the whole I think my boys did benefit from being in a co-ed school, not sure I would have done though.