What you said. There is no extra pressure or anything like that, the only difference is a motivation thing.
So like the person who wants to do well regardless of any other factor will do well in both a selective and a non-selective high school. But aside from this, you get more motivation from a selective school based on the fact that everyone else is trying and you want to keep up with everybody else, so you try that little bit more than you would at a comprehensive school becuase you would have a much higher posiution and a comprehensive school.
But all i am saying is that the opposite can work where it actually helps to be in a comprehensive school becuase it makes you feel as though you work is getting you somewhere. For example, if a student says "ok im going to try really hard this year". At a selective school, they may not improve their rankings very much, but at a comprehensive school they would go much higher, in which case they would say, "ok i like getting good marks therefore i will do this all the time" but at a selevtive school they will say "oh, my extra effort isn't worth the marks i got" and then never try again.
Thats what happened for me...