Buses are not required to have seatbelts because they have a far greater mass than a car, making it harder to, you know, fling people out of a window/into the seat in front of them when a crash occurs, which is what seatbelts are designed to prevent. Oh, and you really think a single bus driver can make 20-30 kids keep their seatbelts on during the ride to school? This is mission control, please come back to Earth.