The reasoning is, the vast majority of school bus accidents involve collisions with much smaller passenger vehicles. Since the bus is much larger, the affects of the impact are minimal. The seat backs supposedly are high and padded to provide adequate protection for the passengers in the seat behind.
As for the fire, I suppose it would be the same for a minivan, however, public schools don't generally use minivans to transport children.
As I understand it, approximately 30 children die each year in school bus accidents. A fairly low figure compared to the number of "child miles" transported each year. My guess is that number is acceptable when compared to the cost of adding seat belts and such.
How many are injured?
I hope your child or grandchildren never become an 'accetable statistic'.