"Should" is doing some heavy lifting here.
Sleep patterns start changing around puberty. Young kids tend to get up earlier, naturally; teenage biorhythms are tuned to stay up later, and sleep late.
But we don't have a society where it's safe to let teens run around by themselves, except for some rural communities, so schedules are based - as GP says - around parent's schedules. And because of workplace demands, whether it's a reasonable requirement because the job demands an in-person presence like the service sector, or because of idiotic, arbitrary in-office policies, that usually means parents need to have their kids in school before 8, or 8:30 if they're lucky, so they can be at their desks by the standard 9am.
Little kids, this is less of an issue, but it really fucks with teenager's biorhythms, because they're designed to be sleeping until 10 and going to sleep at midnight at that age.
There's a ton of studies about this, and there's been a lot of work by K-12 to figure out how to accommodate a balance; and some companies even have policies allowing for flex time to help, but on average - as usual - Corporate America fucks it up.