Driving a school bus is not rocket science. They could have been driven by volunteers.
As for the regular drivers, they were probably out making undocumented purchases with indefinite payment plans at a local Walmart, convenience store, liquor store, etc.
Well, if city had a plan for emergency evac, they would have a list of drivers. Call them in and start evac routes. All Mayor could offer was free bus service to sites inside the city :(.
Also, I heard airlines are bringing planes in for evac purposes. They should have been used as well. They canceled flights and left people stranded. They had no business doing that.