You expel them and they become another person's problem. I heard recently of a local problem child aged seven. He's already been expelled from a private school but has entered a state school where he seriously injured another pupil and attempted to strangle one of the teachers.

Expulsion isn't going to reform them, it will just move it on elsewhere.

So directly to prison. Or must they succeed first?

> expel them and they become another person's problem

True, but we have institutions dedicated to dealing with people like that.

A school isn't that kind of institution and will fail in its mission (to protect and educate) if it tries to fill the role of controlling violent people.