I've actually done some research on this issue and the results are disappointing. Their parents consider the school to be "in charge" when the kid is there and assume all responsibility. They usually have no time at all for their kids and just don't want to be bothered.
So, Send the brat home and force the parents to take a role. There should be a test and permit before anyone is allowed to be a parent.
If it happens in school then parents might to know about it. If their kid(s) are bulling, The parent shouldn't be the only one punishing the kid(s). But it shouldn't be that hard of a school punishment, That's why I think kids should just have an in-school suspension. Thank you!