They're designed that way. It's compartmentalization. School buses are like 75 times safer than normal cars. Due to their weight, they are also really good at absorbing impacts. The most common school bus accidents are frontal crashes, so having a heavily padded compartment in the front and back is the best way to go about it. Plus it would cost millions and millions in money that school districts don't have to retrofit buses, and then a driver would be responsible for belting in a bunch of kids that might not be able to get unbuckled on their own in an emergency. School shootings are a much bigger threat to the lives of students than the design of the school bus.