Apparently, they weren't used because... all the New Orleans Public Schools drivers had evacuated the city. At least, that's what this completely unconfirmed report says:
I sent an email to Alvarez and Marsal (business managers of New Orleans Public Schools) and asked them why they did not make the school buses available to evacuate the citizens of New Orleans.
Their answer:
Thank you for your question. School buses were not used as all the NOPS transportation employees were also individuals who evacuated the city. Thanks.
Of course, others (like those trying to get the heck out) could have driven the buses, but I guess that wasn't considered or would have been against policy...
Once again, this report is not yet confirmed.