I've had that controversy and the one sparked in my city with the bus driver not doing anything in this video:
And in both instances the bus driver did right. Let it flow off the back, either stop for a cop (in the later) or continue on if situation is safe. Bus driver, not security.
I can say, on the bus beating above, I stood with a peace rally in Belleville, IL. They wanted the driver arrested, etc, but it happened at the school. It was up to the cops (which where already standing outside the bus) to get that stopped, but they didn't want to "get in harms way". I feel sooooo safe. I have a feeling it was racially motivated knowing this area, no clue if it wasn't. I actually felt like that dude in highschool, way too many stereo's to tell, but I gotta hand it to the dude that broke it up and got mad.