Even if expelling students doesn't reform them: first, it protects other students; second, ex ante, it's an incentive to behave better in the first place. In fact there's quite good evidence that this works well. (You can check the evidence yourself, by asking ChatGPT "What's the evidence on expelling badly behaved students from schools?" or a similar question.)

They are already societies at large problem keeping them in the school degrades the function of the school for everyone while providing nothing of value to violent students.

The bottom quintile (of stock, not of class) has always been society's problem. What changed is how they're dealt with.

It's society's problem regardless. Not expelling them just means the school itself is an incubator and breeder for antisocial behavior, and when they get out of school, they're still a problem.

These schools certainly aren't reforming misbehaving students.

What do you propose other than “lol” and sarcasm?

Society at large has tools and systems in place to better deal with a violent person, student or otherwise.