One option would be to eliminate dedicated high school buses and instead ensure that there are sufficient public bus routes running at that time to fill the role (with free passes to all high school students). That would cut down on the total number of buses on the road since you aren't duplicating public and school buses, and decrease the number of shifts that the school buses have to run.

Another is to have high schools start last so they can watch their mid-school age siblings while parents go to work.