A recent economist article explains it well:

>Since the 1950s Americans have been moving away from cities to sprawling suburbs with limited public transport. Families became car-dependent and buses were used to get pupils to school. But to save money schools often have to share bus fleets. High-schoolers are usually picked up first, then middle school, to prevent elementary pupils from waiting in the morning darkness.

https://www.economist.com/united-states/2022/06/23/californi...