The 100% decapitation is a bit outrageous. That isn't true. Even in rural areas, school buses do not stop in places where there is any risk of a highway speed collision. Which also lowers the risk of death anyone in a smaller car.

And there are reasons beyond them being built on medium duty truck chassis, that in the US school buses sit that high. Part of the idea is, that if the bus gets rear ended, whatever hits you get deflected down and under the bus. This makes an accident much less likely to injure passengers in the back.