Also, these shorter buses are used for public school for transporting a smaller class to and from a field trip. In Iowa, we also have Open Enrollment, where kids can (and do) attend schools in a district other than their own. As there are usually not many of these kids to transport to/from one district or another, and since often smaller school districts send special needs kid to larger districts, these busses often get used for inter-district transportation. They may also be used to pick up home schooled children for studies in certain subjects and for socilization with peers.
Another user of surplus short buses are daycare centers, as they're quicker and easier to board by smaller children, and often a given daycare center will only have so many kids to transport from a given school.