Teachers/admin can't long-term isolate the shitheads unless they go
really far—notably, it's very hard to be kicked out for bullying that's short of repeated cases of extreme outright violence. A teacher and even school admin can't "fire" a chronically disruptive and/or bullying student. Other students can't vote with their feet and flee the classes with the worst bullies. They're stuck and no-one has the power to fix the situation.
That's why it's so bad. One employee starts pulling dumb shit and several employees report it to the manager, good chance that employee's not going to be able to keep it up much longer or they'll be gone. Yes, abuse happens at workplaces, but school-type bullying doesn't have the same kind of cover it does in a school. Several students report a bully, that bully will still be there next week. And next quarter. And maybe in their class next year. Still being a bully.
Selection bias during admissions gets talked about a lot when it comes to private schools, but they also have the superpower of being able to tell a kid who won't shape up to GTFO permanently. One family's tuition isn't worth risking several other students leaving. This can and does happen, and it doesn't solve all the problems, but it means the worst of the worst don't stick around like they do at public schools, poisoning the whole school atmosphere.