The public high school in my area has classes from 7:35 am until 2:09 pm. The historical reasons for the early start and dismissal times is to allow 2.5 hours a day for extra-curricular activities and/or a large block of time to work part-time, and still have a few hours for homework.
Ours was 7:35 to 3. I don’t live there and have no kids so I don’t know if they still do.