It is also bounded in the other end by sports. If your school day goes too late, you won’t have time in the afternoon/evening for sports practices. So, if you still need X hours for classes, you’ll need to start early enough to get over in time for 1-1.5 hour practices.
(It also applies to other extracurricular sports, but I doubt anyone really worries about play practice schedules)
(It also applies to other extracurricular sports, but I doubt anyone really worries about play practice schedules)