Sports are another big reason parents want older kids to get to school earlier than younger kids (as they can't take everyone at once, or else you need way more busses)—longer daylight hours after school for practice. Any attempt to change the schedule to get older kids to school later and younger ones earlier, will run into strong resistance from Sports Parents™, no matter the benefits to health or academics.

("why don't they just have the sports teams do one short practice in the morning, and one short practice in the afternoon?" 1. some of them actually do that already, despite the crazy-early school start time, and 2. for the less-insanely-dedicated sports that can't rely on parents to drop all the team members off before school, you'd need to run extra busses just for the sports kids, if they're showing up earlier than everyone else, and 3. Depending on the sport, one long practice is better and/or a lot more convenient than two short ones)