The closest you will find are the Arii Bonnet Buses in 1/32. The issue with them is they are Japanese and have the entry door on the left side, as opposed to a US school bus which has one on the rt side. You could probably cut the door section out and relocate it to the rt side though. I have thought about doing the same.
Arii Bonnet Bus at HLJ
Another option is to scratch/convert one using the above-mentioned Opel Blitz kit. There is an article in the May 1990 FSM where Lewis Pruneau did it to show a Cambodian bus covered and filled with riders called, "Standing Room Only". It is on the front of the mag.
Good luck.
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