This is so the older kids can start earlier. There are only so many buses. Adjust one to start later and the other starts earlier, and vice-versa. The high schools should have the 9:00 start and the elementaries should get their 7:40 start or whatever it is, assuming the whole window can't be adjusted.
Nb. I've had it explained to my by people in education that early starts in high school are insisted upon by the sports folks, so they can have longer afternoon practices with sunlight in the Winter, and that there's no hope of switching start times up on a large scale without attacking that constituency and winning, which is... probably not gonna happen.