Good question and I looked it up and here are the reasons given:
They have designed school busses in a way that compartmentalizes the kids so if they are thrown it’s about into the back of the seat in front of them.
If they did have seat belts the driver wouldn't be able to ensure all the kids wore them and the kids would cut them out or choke each other with them.
On average 10 kids are killed each in school bus accidents and most of these are collisions where "seating position was in the direct line of impact" (side impacts) where a seat belt wouldn't have provided much help.
To put that into perspective 40 to 50 thousand people are killed in auto accidents per year and several times more kids are killed walking to school and boarding/exiting a school bus every year than are killed on a school bus.
It is possible having a seat belt would have helped the kids that went over the overpass but we can’t afford to equip every school bus in the country for such a remote possibility. It would cost $470 million dollars to put seat belts in school busses (1,500 per bus * 384,000,000 school busses) and that would ensure the kids wore them and didn’t cut them out. We would have to hire an extra employee for that AND what are you going to do when the buss runs off into water or catches of fire and the kids are buckled in?