I remember we had something like this happen a couple times on my bus route when I attended high school. Except that it was because most of the students were just douches. It never went as far as going back to the school, most of the time, they would just sit at the school, and have the principal/VPs come to the bus, or pull over on the road if we were already driving. I agree with Firefly as well, bus drivers have a tough job, and I really felt bad for most of them who drove my high schools routes, because teenage kids are just lacking any control. I remember times were people would throw stuff out of the windows, at other buses (we once had a yogurt fly through our bus window from another bus), People standing up, yelling, etc.
Do they have cameras on your buses there? I know that here, they decided to install security cameras on the buses, so if something like this ever happened, they have the footage to see what really happened. Plus, we also have undercover cars, that will follow the buses every so often, to make sure the bus driver is doing their jobs right/everything is going the way it should be.