One of the problems is that kids in the top streams in highly-selective schools do very well. The kids at the bottom often don't so as well, and sometimes become problem cases. These same kids -- who are very bright -- would be stars / top students if they'd gone to a lesser school.

Source: Personal experience. I went a grammar school in the UK. I also taught in a top UK grammar school and a not-very-good comprehensive school. I've seen it happen numerous times.