I think it's a fine idea. The schools need money. The kids have already been marketed to and poisoned with soft drink machines right inside the schools (you can't get worse than that).
The schools could charge a lot for ad space on the school buses - who wouldn't want to be seen as a supporter of education.
The city buses in Minneapolis already have ads on them. What makes school buses any less?
(LOOK !!! I could support education, too !!!)