I agree with every word of that, Stickerrocks.
If I was a teacher, I'd want to do good, but I not a living saint so if I got abused on a daily basis by students, SLT, parents, the media, politicians etc, and all the while I'm struggling to actually teacch, I'm fairly certain I'd decide to either use my degree elsewhere or get a job in a school where behaviour is less of an issue.
What i was getting at though, is that one or two disruptive students ruin things for everyone, so maybe rather than filtering out the top 25% by ability, it would be better to filter out the bottom 25% by behaviour and apply comprehensive teaching principles to the other 75%?
What you'd do with the disruptive segment though, i don't know. It seems to me that no one knows and that's why they leave school with low prospects now?