I was lucky to stumble upon School Bus a couple of weeks before my son was due to take the school bus for the very first time. The book is very graphical, with minimal words, but manages to convey the whole process of getting on the bus, arriving at school, and then taking the bus home after school — from the perspective of the bus.
He read it at least four times the first day I borrowed it, and I think it helped him understand the whole concept of taking a school bus.