So seatbelts would save lives, but they are too expensive, right?
MotionMan
And then there's the additional expenditure of one bus driver buckling in every single student for every trip. And you also need to somehow ensure the belt stays buckled throughout the entire trip.
Even then, they'd only protect against rollovers, which are very rare, and provide partial protection at that. There's also the risk of death if one or more of the belts gets stuck.
Seatbelts in school buses seem like a good idea, but they really aren't.