Apparently they are "less comfortable" than the commercial buses.
http://www.cnn.com/2005/US/09/04/katrina.impact/index.html
quote:
Those evacuated Saturday were either put on commercial buses and taken to Arkansas, or put on less comfortable school buses and dropped off at New Orleans' Louis Armstrong International Airport. They will be flown from there to Arkansas, Federal Emergency Management Agency officials said.
http://www.cnn.com/2005/US/09/04/katrina.impact/index.html

