Still to this day I don't know why buses don't have seat belts
This video took me down a rabbit hole on this exact question an hour ago.
Apparently , they arent needed due to the mass of the bus itself and the higher height of the bus relative to most traffic.
it seems like a colliding object is likely to go under the bus , instead of through it. Additionally, the belts would cause more harm than help, as they would twist the spines of children in such a collision. As for mass, the intertia of the bus allegely keeps it stable againt collision due to how much more energy a heavy 30mph bus has against a 30 mph car or pickup truck. Picture a feather weight boxer trying to tip over a sumo wrestler with a jab.
I think an incident like the one in the clip is a rare outlier, cause buses are statistically unlikely to be hit by a 100mph object in a lowspeed area. Such a statistically unlikely scenario would have enough inertial energy to overcpome the mass of the 33ton bus at a low or stationary speed. In order for a tiny mustang to tip over the bus it would have to have been at a near standstill for the bus to tip over like it did . Roads simply arent designed to have stopped objects like a bus in the same area as hundred mph objects , thus inmost cases the bus would be left standing.
As a kid, i doubt busses ever went beyond a 40mph street. And even in the rare cases of field trips on the highway, the inertial mass calculation would still be in the favor of the bus cause a 33 ton bus going 60 mph, would still have dramatically more inertial enrgy than a 2 ton mustang going 100mph allowing it to be realively unscathed in a collision.
Because of the above listed reasons only 100 people die each year from school bus accidents, while 20,000 people die in car crashes. And keep in mind that 70 percent of the bus related deaths were from people outside the bus.