Early school hours are often a result of teachers' desires to get out of work early. Many teachers' unions include school hours in their bargaining/contracts.

Yeah, and that is because being a teacher is not just an incredibly mentally taxing job (made worse by the fact that class sizes are way too large) but also involves an awful lot of invisible after-school work: preparing and correcting exams, preparing class material, dealing with IEPs, following up with parents (particularly in financially or otherwise challenged families), organizing after-school and extra-curricular activities, dealing with other bureaucratic bullshit because the administration is understaffed...

Yes, it is commonplace for bussing to be staggered -- they often pick up a route for one school (maybe a high school), then run another route for another school (maybe an elementary school).