Expelling difficult kids has the nice property of aligning those parent's interests with everyone else's.
If you have a difficult kid, right now if you don't handle the problem it becomes everyone else's problem but you're not affected.
If the difficult kid gets expelled, now it is solely your problem. It only takes a few cases for parents of difficult kids to become extremely interested in the behavior of their kids and educating them on what proper behavior should be. As it should be.
I'm not sure that actually puts pressure on the parents to be better. I think that decent parents generally raise decent kids because they're good role models, and the opposite for bad parents. There are exceptions, but I wouldn't rely on it. If a bad parent suddenly has to be fully responsible, I doubt they'll suddenly have a change of heart and be able to change the kid's behavior (depending on how old the kid is).
