I am an automotive restoration specialist, and have thought about the subject of seat belts in buses many times. The cost of retro-fitting and installing seat belts on buses would be enormous in the extreme.
Yeah, but they could make seatbelts a feature on school buses going forward and grandfather the old ones. They didn't have seat belts on school buses when I was a kid, either; but if they'd started making them standard on new buses then all buses today would have them now. Gotta start somewhere.
Buses get replaced so often it would be easy to get them all equipped with seatbelts. If they start ordering them with seatbelts Today.
I used to drive a school bus... 60, 70, sometimes 80 kids under 12. You do not want to have to go around undoing seatbelts on 5, 6, 7, or 8 years olds who are panicking should the bus catch fire or you get stuck on a railroad track.