You are just pushing the problem down the road if you have an increasing number of violent kids and expel them all from school-- how are they ever going to become productive members of society? You just end up with society either paying for them being in prison, subsidizing them (because uneducated and unmotivated labor is close to worthless) or paying to literally clean up their shit because you basically routed them into a "careeer" as homeless person.
If parenting standards have slipped so much that violence in schools becomes a significant problem, you need to either fix this by getting the parents to do better or by having the education system itself patch those deficits; any other "simple solution" is just gonna result in immense costs to your whole society that compound for decades.