You’re assuming a perfectly rigid system. The school bus has a lot of mass in front of the rear wheels and under the hood. But behind the rear wheels, it’s basically just aluminum sheeting bolted onto the frame, and children sitting in seats, packed in like sardines. That means the mass at the front of the bus is actually working against you, because you run the risk of sandwiching all of the kids at the back of the bus. If the tail of a school bus crumples, you’ve just shattered 20 kids’ femurs and the fire department is going to spend the next 6 hours cutting them all out of the wreckage. The ideal method is to direct as much energy away from crumpling the frame as possible. And the best way (aside from adding a cow-catcher wedge to the back of the bus, to fling them off to one side or the other) is by turning it into vertical force that lifts the tail, instead of crumpling directly into it.

A normal car doesn't have a bumper to bumper collision with a schoolbus. My car would have a bumper-to-headrest collision at that speed.

A bus has a lot of mass in the front, the rest of the bus not so much.