Can you define "work" in this instance?

For example, if they made half the kids do better, and half the kids do worse, does that count as working? While you sometimes get claims that it's for their own good that less academically inclined pupils are sent to the non-grammar schools, I'd guess the same people who say this would freak right the hell out if their own kid failed the exam and didn't get in, which kind of undermines that argument.

I meant that they work for the individual, a child sent to a grammar school will get a solid education.

I take your point about the wider ramifications, but I feel as though I got a good education, and if I ever have kids, I would want them to go to a grammar school too.

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.