Then you follow the rules as you were told. Stop the bus, call the school. You don't just keep driving.
If the parents are mad because the kids were late, who cares? You did the right thing. Their little monsters didn't and yeah, I've dealt with enough of these parents to know that they think their children are entitled. That is why you follow your rules and the problem is theirs, not yours.
I've never seen it get bad enough for the school to tell the kids they had to find another means to get to school. I don't know if they can. Mostly it will be expulsion or alternative school. But, this kind of behavior needs to be stopped early on. Keep the kids who pick at each other at opposite ends of the bus.
you clearly dont know much about what its like to ride a public school bus or be a bus driver in a city