Teachers spend a non-trivial amount of time post class working on previous assignments, future assignments, setting up tests coordinating with other teachers and staff. If they start all this at 5, they’re stuck at the office until very late.
Busses/kids on the road before rush hour
Extra-curricular activities are better off earlier than later, don’t want clubs running into diner time.
better chance of getting home before dark in winter at Northern latitudes