Many conservatives (and trolls) have tried to make political hay out of the proposal that Mayor Nagin should have used the 200 school busses they claim he had at his disposal to evacuate the 80,000 people left behind in NOLA. So let's do the math: 80,000 people and 200 busses works out to 400 people per bus. A typical school bus holds about 40 people. That means that every bus would need to make ten trip trips to... somewhere... to rescue the people.
Now remember, they have to be taken somewhere. Let's say the busses go to Baton Rouge, 80 miles away. That's a one hour and forty minute trip (according to Google). One way. Each bus must then make the 3.5 hour round trip ten times. But of course there's loading and unloading, waiting for people to arrive (and the question of how to get people to the busses), the time it takes to fill the busses with gas for all these trips. So let's say it's a six hour trip on average. That means it would have taken roughly 60 hours, or two and a half days of constant travel to get the people out. But wait, all this had to happen in the days leading up to the storm, right? The days when the roads were clogged with the rest of the population who had access to private cars. There's no way to tell how long each trip would have taken under those conditions -- twice as long, three times? Who knows? And this is assuming Baton Rouge had enough shelter for 80,000 refugees (they got hit by the storm too).
This entire logistical operation is supposed to have been carried out flawlessly by a single mayor with limited resources at his disposal, no disaster training and dozens of other tasks on his plate.
Moving those people with the resources at hand was a logistical impossibility.
Moving them to available shelter within the city was the only possible solution at the time.