I'll get shot down in flames for this, but I'm a tough old bird and don't mind going against the flow.
I think the concerns about single-sex education are misplaced. From my personal experience and observation as a pupil, parent and teacher over the years I'd pick single sex every time - until say, sixth-form, when youngsters are hopefully more mature.
It depends what you want for your children. If social life is the most important thing, I suppose there's a slight argument for mixed sex schools, but I had a great social life at my all-girls grammar back when dinosaurs roamed the earth.
I remember friends whose children went to mixed schools moaning about the ridiculous pressure on them, even from Y7, to be attractive to the opposite sex. Girls and boys would make themselves late for school by spending time grooming themselves and arranging their hair, make-up, dress in order not to be picked on.
In single sex schools this pressure is largely removed - they can save all that for the weekend!
I just think that kids focus more on their school work and activities when in single-sex settings. The idea that, in this day and age, they somehow become incapable of dealing with the opposite sex is a bit outmoded. They can join clubs and hobby groups and meet boyfriends/girlfriends if that's important to them at that age - it wasn't for me. I was happy to wait until sixth-form for all that.