I went to an high ranking academic all girls independent boarding school.
There were lots of pros and cons. Some relating to the fact that it was all girls.
Many of the girls did not know how to relate to boys at all. I have 2 brothers, so that wasn't an issue for me. It meant that if there ever was any socialising with the boys school (very, very rare) the girls went a bit mad, with clothes and make-up etc.
The boarding houses could get quite bitchy at times, but that was more to do with how they were run than the fact that it was all girls, and I don't think the same applied to the classrooms during the day.
On the academic side, I think there were definite advantages. We were very focussed and the underlying unspoken expectation was that we would be achievers, high flyers, the next movers and shakers in the world. There was no sexism, no gender put downs, no insults and snide comments relating to 'being girls' Most of the teachers were real blue stockings, and so there was this amazing sense of what was important (school work obviously!) A bit Miss Jean Brodie in a way.
That sense of being as good as/better than boys was very important. It was a great feminist place actually, although I am not sure most feminists would recognise it at such.
Because my experience is single sex and private, I can't compare it to a co-ed comp, but I have imagined (probably wrongly) that the girls have to fight harder to prove themselves.
Ds will go to a single sex boys grammar in sept (I hope) He would prefer a co-ed school, but the only grammar we can get to is a boys one. It is next door to the girls grammar. They have a joint canteen, joint orchestras, theatre productions etc and then a joint sixth form. They can talk over the fence on the field at lunch time! I like this, it seems like a good balance. I like the fact that in the boys school there are lots of make teachers, good role models and lots of people who like boys and understand them (don't think our education system always does that, but that is another thread) My dd will try and get into the girls school. I like the fact that as a quiet but bright child, she will get some space to blossom, where girls are highly valued.
But dd2 would do better in the boys school! She doesn't like girls and all her friends are boys. She is only 6, but I am wondering what we will do with her, as I am not sure a single sex school would be the best.
Ultimately, having just done the agonising selection of schools for ds, I would prioritise the atmosphere of the school, the results and the behaviour over the fact it is single sex or not.
Our second choice school is a great co-ed comp, with high standards of behaviour and good results. Although I like some of the things about the grammar being single sex, it wasn't the single sex issue that decided us, quite the reverse, we would have preferred a co-ed Grammar.