8:30 or later. Good. I say start it at 10. Let the kids sleep in and be happier for it.
1) The flipside to starting much later is that kids will be exiting schools later. Want to participate in some after school activities or clubs that aren't physically in school? A HS kid wants to hold down a little part time job so she can save to buy that first car or afford that summer trip she wants? Any activities out of school become much more difficult to manage if the exiting time moves any later. And these extracurriculars are also a big part of many peoples' lives.
2) I think it's naive to think that kids will get much extra sleep regardless of the school starting time. Their need for socialization and leisure activities out of school doesn't suddenly vanish if school starts later: they just get shifted. If you tell kids that they can manage to wake up at 9 instead of 7 and still make it to school on time, they'll just tend to stay up about 2 hours later playing games, watching movies, or chatting with friends. The end result might be about the same amount of sleep.
3) Logistically, it's hard enough for most parents to see their kids for a few minutes and deal with getting their kids out of the door in the morning (or driving them to school) when their wakeup times roughly coincide and they can get a little breakfast together or have a few minutes to chat. Trying to arrange a much different school schedule for kids is going to present a lot more difficulties to working parents, particularly if the kid has some special needs, is a tad rebellious about going to school, or you have to drive them for some other reason.
Just thinking out loud here, but I'd say that getting rid of the nightly busy work (homework) is a better way to fix teens schedules. They already spend enough time at school learning. Stop forcing them to waste an hour or two every night regurgitating mostly busy work and then feeling like crap and needing to stay up another few hours decompressing. Let them get their fill of fun and socialization after a long day. Without the burden of homework, most teens have enough extra time in the day to get a little extra rest and still have enough to unwind and feel like a human.