These parents owe no one an apology.
Only because it's allowed. Replace 'public or private school' with 'workplace' and see how ridiculous it sounds. This doesn't commonly happen in adult environments because the result would be job loss and/or lawyers and financial penalties. If the schools were really serious about bullying they'd get rid of the problem children but they're not, nor are the school districts or the parents of the bullies.
You and I wouldn't shrug and say 'well, that's the way it is' if our co-workers bullied us but it's accepted among children as uncontrollable.