I did, once, see an article that described the decision to not install seat belts in school buses (probably about 40 years ago): The kids won’t wear them anyway; the driver cannot be expected to enforce wearing them; Buses would need to be retrofitted with better fastener points because the existing floor isn’t tough enough, the seats would also probably come loose in an accident, making it safer for the kids to be thrown in the same direction… AND it would cost a lot. They had also run some kind of crash test with a standard school bus in order to come up with the information about how cardboard-like school buses are.
I did, once, see an article that described the decision to not install seat belts in school buses (probably about 40 years ago): The kids won’t wear them anyway; the driver cannot be expected to enforce wearing them; Buses would need to be retrofitted with better fastener points because the existing floor isn’t tough enough, the seats would also probably come loose in an accident, making it safer for the kids to be thrown in the same direction… AND it would cost a lot. They had also run some kind of crash test with a standard school bus in order to come up with the information about how cardboard-like school buses are.