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 becom

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.