Not that rare. When I was a kid (in the 90s) I lost two classmates to bike crashes. One actually dead, the other severely brain damaged for life.
Neither accident involved a car.
Both would have likely had a few broken bones at the absolute worst if they’d been wearing helmets.
I wasn’t in an especially large elementary school, either.
I'm sorry for your loss, but your school is very much an outlier; public policy should look at the overall rates, and statistically it is rare. Particularly if we're talking about an everyday school commute along surface roads (the risk profile for mountain biking or BMX-style stunt riding is quite different, and wearing helmets for those activities makes a lot of sense).