Everyone has a story to tell, and I like reading about different experiences. In general, nearly everyone comes out in the wash, and I agree with sazzl, and others, that a bright child usually does well whatever type of school they attend.
I went to a rough London comprehensive school in the 70s. I was "clever", but never saw myself as highly intelligent, though, not boasting(!!), I did rise to the heights of being Head Girl, achieving excellent A Levels and going to a top university! This was alongside bunking off from lessons, hanging out in the local park, and witnessing undesirable behaviour from schoolmates.
Anyway, I also happened to be part of a huge nationwide government survey, NCDS, at the time, which is on-going even to this day. I was deemed "a gifted child" in an area unrelated to my schooling, and had to attend the offices of a psychiatrist in Harley Street for assessments. The report concluded that my potential was way above my attainment, and that, academically, the comprehensive education wasn't stretching me fully. I have no idea whether this was due to lack of resources, lack of expertise, lack of expectation or whatever, but it was failing its pupils.
I wonder how many slipped through that net, and how many would have been better off in a different school environment. One size doesn't fit all, and there should be different choices for everyone. Success is very often down to luck, as to the schooling available in a particular area, and of course, down parental input.