When we moved to the UK my eldest was 14/15 yrs old. He had no choice but to go to the local comp. By that time though - it was already ingrained in him as to what type of a person he was. He was bright, he would've blown his way through the 11+ had he been here to do it. He would come home from school initially saying 'what a waste of time, no lessons get finished because the teacher never gets the chance to actually teach'. Well, he got to know his teachers, I got involved with the school and got to know his teachers. My son made it clear that he wanted to succeed and come out with great grades, and he wanted to work hard at it. He got all the support in the world from his teachers, was usually given an additional challenge and came out having done wonderful after A levels. The one area they did fail him, was in first year university he was diagnosed as dyspraxic......when speaking to one of his teachers last year and having been asked how my son was doing, I mentioned this to him. His comment? 'oh yeah, that's right'......when asked what he meant by that i was told that 'I couldn't say anything, as the school would've had to have taken steps, but we don't have the funding'. So......yes, a child can do very well at a comp if it's in their own nature to avoid the pitfalls, but I do think that without a statement, any extra needs that should be met just get overlooked.
I once had the headteacher say (at the comp) that they couldn't exclude certain students for their appalling behaviour, as the parents would likely 'sue for disallowing their child an education'. I warned him at that time to be very much more afraid of parents like me, as I could certainly sue them for disallowing my child a 'safe learning environment' due to tolerating some of the behaviours that they did from other students.
You know your child, you know if he would be misled by others, or would go with what he knows is right/wrong etc. I think that we all need to give our kids themselves some credit in choosing where they feel comfortable when we are looking around at the schools.