Earlier quoted context omitted.
The generalized version of this is "take away something they care about". But it's not always easy to do. In many cases, schools have nothing the kids care about. If they do, rules often prohibit them from using it as leverage. And in many cases parents also are unwilling to apply any kind of consequence that would make their kid unhappy.
Expel the kid I want everyone to succeed as much as possible, I feel bad for such kids. But at that point, the kid won’t learn, won’t launch, there’s no benefit to keeping them in school and massive consequences for the good kids.
1) school education is mandatory until 16-18 in most countries, so what do you do with them once they get expelled. They have to be in education somewhere - so do you just put them in one school for all the expelled students, which is just constantly on fire? You made the problem much worse for yourself(as in - the state).
2) " there’s no benefit to keeping them in school and massive consequences for the good kids" - the massive consequences for kicking them out and not dealing with the problem are then on us, the society, because you get dysfunctional kids that got no help and just got kicked out instead. What kind of adults do you think they will grow into? Or is the answer "I don't care"?