If the driver - disarmed by policy AND probably the law - had intervened, he may well have been another victim.
Assuming he wasn't fired, but left on the job, either the perps or their homies would have targeted him, knowing his route, his schedule, and the fact that he was disarmed.
If Trayvons 1, 2, or 3 had an injury - real or imagined - civil suits against the driver would result. And the school district would fall over themselves throwing him under the bus - he'd be entirely on his own.
Being given "responsibility to the kids" is meaningless unless it includes both the authority and the means to exercise that responsibility. In the case of a school bus driver, that means safe driving. Period.
It's a sad commentary on our society, but I have a hard time faulting him when every likely consequence to his intervention would be negative.