Well I'd say New Orleans to Texas is a pretty far trip by school buses. I don't think you could make more than one or two round trips in a day. That would still leave a lot of people left. And such things as fuel and sanitation would be issues since school buses are not made for cross-country trips.
I think the mistake was is not having a levee system strong enough in the first place. When you have a levee that is designed for cat-3 storms in an area that is subject to cat-4 and cat-5 storms- this is a disaster waiting to happen. So I say forget busing and get those damn levees strong enough to do the job. If you can't- then people shouldn't be living there.
Every time I've ever built or bought a house I've gotten a USGS topology map to make sure it was not in a flood plane. I wouldn't even think of living any place that had a history of flooding, much less in a place like New Orleans where the only thing between you and 15 ft of water is a pile of dirt.