School bus driver "vetting" is checking their driving and criminal record and "training" is essentially how to operate a large vehicle. The first could still be done, the second doesn't apply.

And the liability issues all seem to go the other way, don't they? Buses are less safe (they don't even have seatbelts), problematic incidents involve a larger number of students, the buses are more officially associated with the schools, etc. And actual parents obviously have better incentives to make sure their kids are safe than someone who is only doing it for a paycheck.