I certainly don't have dull children and they went to schools like Habs, North London Collegiate etc which are very academic, very mixed and not posh and are day schools with loads of relatively poorish children there with all family money from the corner shop usd to buy the place etc.
My second daughter at university in tutorials said it was amazing how little the state school pupils said. For a start many were not confident. They seemed to have little experience of debating. Many said um and ah and like and didn't have very good English language skills and they just didn't seem to enjoy debates. They sat dumbly there.
Secondly my 3 are leaving university and I notice that people from privage schools seem to be better all rounders, accent better, confidence, better and most of all higher expectations. The state schoolers think being a teacher on £30k a year is a marvellous achievement or a secure £20k a year Government job is truly wonderous. So they may well have a good university degree but they aren't much good at doing anything decent with it.