Driving a bus is not rocket science. Even the 19 year old kid could do it and he'd never driven a bus in his life.
No government official is going to give just anyone the keys to a bus full of indigents and elderly -- especially with the roads so tangled with traffic headed out of town.
There really was no practical way to evacuate New Orleans. Like the cops told the people, it was "every man for himself." Of course, that was a recipe for disaster in a town full of welfare queens and thieves.