Class size doesn't matter as much as class makeup. They need to put the smart kids with the smart kids. That way, you aren't dragging down the achievers with the underachievers. Don't place ceilings on achievement. Put the bad kids with the bad kids so they don't drag down the other kids.
I agree that class size isn't as important as it's assumed to be. Small is better, but not necessary. And I've even seen different achievement levels work in one classroom, although I think your overall point is a good one. I sat in one class where the high achievers were given an opportunity to learn the subject material quickly and take a test near the beginning of a unit (this was an 8th English Grammar class, so some students were able to do this). They were then placed at the back of the class at a row of computers where they could work somewhat on their own with some teacher supervision on other, related projects. It was fascinating to see this operate. I think it also gives slower students something to work toward. But the teacher has to be very skilled to pull this off. And this teacher was.
"Put the bad kids with the bad kids"
Yes,but pity the poor teacher that gets stuck with a class of thirty of these bad actors.