So if you punt the highschool bus to later, you are either making for a very late school day, or making younger kids wake up earlier.
Or the solution could be to triple the cost of school transportation.
Yes, let's try to kill the one form of public transportation we have in the USA that works decently and is widely available.
Where I grew up it was the opposite. Grade schoolers started first at 8:00, middle-schoolers at 8:30, high-schoolers at 9:00. As a high schooler you finish school at 4PM, hardly a very late school day. Younger kids naturally go to sleep and wake up earlier than teens anyways, it's not unfair or harmful to have them start school earlier.
Sometimes older siblings contribute to childcare. A 9th grader that is home at 3pm, can watch a 4th grader that is home at 4pm until parents arrive home at 5pm.