The real problem is that school personnel don’t want to deal with the parents of the actual problem kids, so they get away with it even under zero tolerance.

^ this. Every time the school staff have to deal with THOSE parents, yet again, is a huge stressor and they will try to avoid it if they can, but the parents of the "generally-good-except-this-once" kids will generally be totally reasonable and easy to deal with, even if injustice is occurring.

How hard is it to just expel them? They seem to expel victims with ease...

That's because the narrative in the last decades has shifted towards tools, not actions and intentions being good or bad.

In the past, it was normal and encouraged to use any tools available to you to defend yourself. Psychological abuse is still abuse and you have the right to defend yourself, the most natural, available and effective immediate defensive tool being violence.

In recent years, violence has become a massive taboo. It's a tool that is universally labelled as bad no matter the circumstances. Instead, everyone is encouraged to portray a "good victim" by demonstrating helplessness and waiting/hoping for people in positions of power to help.