Set Harth wrote:
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?
Interesting you pointed that out. The sound of children was added in post production as no children were on any of the buses.
Back to the film, one of the robbers said that "School's out, it's time to go." So the robbery was planned to the minute that the school buses were going to pass that block