My DD is just going to be doing her exams next week for a girls boarding. It is very difficult to just rely on hearsay because you have to find a school which has to be more like a second home for your child. I saw a few school and its very hard to say why we disregarded some of them (we instantly knew they werent right). The one we ended up with on our shortlist was one we felt would not only enable my DD to succeed but to nature her too. You really need to find one with ethos you believe in. Its more about gut feelings too. The school we ended up with was one which was last on my preferance list but something just did it for us when we visited and met the other girls in the school.
Dont immediately rule out the ones that arent in the top league as WA or CLC, they would have systems in place for the academically able to be stretched. You may just need to find out how they stretch them when you visit or call them. I have some scholarship papers for WA and its really hard and shows they have a high calibre of candidates so your child will be one of the many who are clever whereas for some other schools she could be the one among a few if that makes sense.
Personally I would look for a boarding school which places high value on pastoral care as just as they do on exams too.
Is it 11+ or 13+ you're looking for and is it for 2011 entry?
WOG why do prefer coed? I thought until GCSE single ed is better? One of the co-ed I saw I immediately disliked it. The boys and girls live in the same house and only have separate dorms but share everything else. The other co-ed was diff though boys and girls had separate houses. I am still not sold on the benefits of co-ed over single sex particularly in the early years esp for girls. 6th form is a diff matter and I wholly support it then.