Where I grew up in the States, the elementary, middle schools and high schools shared a transportation department. Because of this school start times were staggered as buses would pick up highschool students and drop off at school, then make runs to pick up the elementary and middle schools, again staggering to accommodate different start times.

This meant that students at any of the schools could get dropped off anywhere from 25 minutes early to just before classes started (from 7:30-9:00am.) The end of the day was also staggered, with school getting out between 2:30-3:30.