On Wednesday morning, Aug 31, Blanco's staff stopped trying to arrange schoolbuses for the evacuation. They believed that Fema was sending them buses. Not only that, they thought the military would use Chinook helicopters to airlift people out of the Superdome.
Then, when that didn't happen by the afternoon, Blanco started the search for buses again, and in the evening she issued an order allowing local school board buses to be comandeered.
This is from the recently released documents, but it's been discussed before. See Sep 19's "Blanco: where were the 500 FEMA-promised buses?"