Ironic how seatbelts aren’t mandated on public forms of transport like trains and buses, yet one is required to wear them in one’s own vehicle. If it truly is based on safety then it should be an all or nothing approach to requiring seat belts on moving vehicles.
Transit has the same advantage as a back seat of private auto: a big non-windshield to crash into in the event of a crash. seatbelts are pretty great. the proliferation of airbags is the correct comparison.
The mass of a bus or train will happily cleave through wayward objects, so there will be less acceleration of your face into seat backs, poles, and other passengers. School busses to my knowledge do have padded seat backs, plus they're also up high, so they're more likely to go over sudden obstacles, increasing the stopping distance.
