There are no federal motor vehicle safety standards covering the installation of seat belts on large school buses. Children have been injured by seat belts used as weapons by other students, and by catching their fingers in the buckles or tripping over loose belts. By virtue of the fact that school buses weigh eight to ten times more than a typical automobile, in a collision the energy of the impact is absorbed throughout the greater mass of the school bus and less of the crash force affects the school bus passenger. It is a well know fact that excess speed kills. That's why school buses trypically travel slower and are highly regulated as to how fast they can travel.
Speed is a factor in a high percentage of automobile accidents.
Thus, the effect of speed in highway fatalities in cars vs. buses isn't an equal comparison. Students are protected by a lower impact zone. School buses are designed so that students sit above the impact zone where automobiles typically strike the school bus. The impact zone for passenger cars and vans is much lower.