Yup. Money for aides is instructional money (principal's budget). Money for buses is transportation money (district budget).
In addition, the district would be assuming huge liability to allow a single adult to walk with a single child, unsupervised. (This is both for the protection of the child and the protection of the adult.) So the principal would actually have to budget two adults to accompany a single child to and fro. (You would think this would come up in the case of a bus driver, too, but at least you can time the departure and arrival of the bus, and have more confidence that there was insufficient time for anything untoward to occur.)
