Right, I'd really like to know about the "random" process that assigned students to classrooms. I know that when I was in elementary school the assignments were theoretically random, but if parents really wanted to influence them, they could. Since this study gets a lot of its results from clustering, they might just be finding that certain classes were perceived to be better (right or wrong) and so involved parents influenced the school to get their kids in it. Now it really is going to be "better" by the metric of those kids' later success.