Basically, the district needed to make sure that elementary school kids weren't walking to school or waiting for buses in the dark, so they had to start around 9a at the earliest. Since they needed to share the buses, they couldn't start all grade levels at the same time.
The reason why they went 725a, 750a, and 9a instead of something like 8a, 9a, 930a is that if you started Jr high or High school at 930a, they wouldn't start after school activities till 4p and would go after dark in the winter.
This always seemed an insane argument to me, but was said multiple times. My home district looks to still use the same bell schedule too.