That route changes every year and not everyone lives where you can expect kids to be able walk to a high traffic bus stop.
If you had proper public transport that number would be quite negligible. Very very occasionally you see dedicated school buses in rural areas in Germany: Minivans. Which makes sense as boondonks areas might not have bus service but have collect taxis as only public transport, which generally are also minivans. Think living in a village of 50 and going to a school in another village, population of 2k or so. The scale of Wacken, maybe a bit smaller. Let me see... Yeah Bokelrehm doesn't have a bus station. OTOH it's like a kilometre from the school in Wacken so kids are probably biking ("Grundschule Wacken", northern end of the village on the road to Bokelrehm).
Usually the most that happens is that a regular bus service gets a doubled-up schedule when school starts and ends.