Also buses are generally much more massive than the vehicles they collide with, leading to much lower forces on bus passengers than on car passengers.

I don't know if this applies to general passenger buses or just school buses, but in the US they have closely spaced seats with energy-absorbing seat backs that limit the force on each passenger and how far the passenger can move in a collision.

Smaller school buses (under 10k pounds) are required to have lap and/or shoulder belts at all seating positions.