You are wrong on multiple points, OU.

Teachers and principals are not strangers to students, nor can they inflict violence on anyone.

Expulsion is usually the least desirable outcome.

Everyone has the right to lay hands on someone else if that person is likely to harm themselves or someone else, is clearly out of control, etc. As a teacher who spent at least 6,000 hours on the playground, if I saw an aggressive kid/bully going after another kid and I failed to intervene, I'd expect to be faulted for my inaction. Teachers not only have the right to intervene, I'd argue that there is often a duty to do so.