Perhaps some stats would help to reassure you and put school bus safety in perspective? I jotted down some school bus safety stats during a lecture on school bus safety at a CEU conference for CPSTs a couple of weeks ago. Let me find my notes.
23.5 million children in the U.S. ride the school bus to and from school and to and from school related activities daily on approx. 440,000 school buses. There were 7 total child fatalities on board these school buses in the U.S. over the course of one year (sorry, my notes don't say which year) and 4 bus driver fatalities, There were 800 child fatalities in passenger vehicles operated by parents/caregivers who were driving children to and from school only, not including any driving to and from school related activities, over the course of the same year.
800 kids > 7 kids + 4 bus drivers, although losing anyone to a crash is regrettable.
The other take away from the lecturer was to ideally ride in the middle of the school bus rather than in the first few rows or in the last several rows closest to the back of the bus, to be farther away from possible frontal or rear impacts, but to again bear in mind that the school bus is statistically much safer than riding in a regular passenger vehicle overall and someone always has to ride in the front-most and rear-most rows on a full school bus anyway.
Does that help put things in perspective?
I know stats like this did for me when DD1 started school and began riding the school bus, and this lecture helped reinforce the decision to let her ride the school bus instead of driving her to and from school daily. My only qualm now is that DD1 is assigned the seat right behind the school bus driver due to her food allergies, rather than a seat farther back on the bus.