I understand your concern. Buses are the safest form of ground transportation. Why? Buses are larger and heavier than most vehicles, crash forces are distributed differently (evenly), the occupants experience these forces differently, they are statistically the lowest "risk" mode of travel, and they are conspicuous as hell.
In addition, buses are designed with the theory of "compartmentalization" which combines energy absorbing seat backs, and narrow spacing between occupants which offers a "compartment" size space for the occupant. It provides passive protection. Think of an egg in a carton.
With that said, there is a push by NHTSA and others to mandate seat belts in buses. When or if this will become law, I don't know. There is a small percentage of bus companies/school districts who use/manufacture buses with lap and shoulder belts in them, right now.