Re: "The Dark Knight Rises" Review and Discussion Thread (spoilers)
It's possible the buses were controlled by The Joker and the line of them was a bit of a "Where's the red card?" trick being played to confuse anyone who might follow him to his destination. There's little to indicate these were active school buses doing regular duty.
The sound of children, maybe?
"Your advertising's just dandy. Folks'd never guess you ain't got a thing to sell."