tbh, i doubt the time that students are required to be at school has much to do with their ideal sleep schedules. only about a third of women are stay at home moms these days, so you can consider parents to fall into one of two groups: shift workers and 9-5 office workers. shifts can start/end at any time so they can't really be planned around at the level of an entire school system. so by default, the schedule gets planned with the assumption of both parents working 9-5s. if both parents have to be at work by 9:00, the kids need to be at school by 8:00, so the parents can drop them off or at least make sure they get on the bus.
also, a lot of school districts only have enough buses to transport all the lower, middle, or high school students at once, so their arrivals must be staggered.
given that school also serves the purpose of gov't sponsored day care, i'm not sure what they could really do better aside from granting tardiness allowances for students with bona-fide sleeping disorders.