I am going on 2 decades removed from my American high school, so maybe things have changed. But I also recall starting school within +/- 10 minutes of 8:00 AM.
The only exception was strictly extracurricular, a men's choir which practiced between 7:00 and 8:00 AM. The expectation, implicitly, was that parents or friends were going to be the ones getting you there.
I've never heard of a 6:15 start time.
Also, school was never a full 8 hours. So I'm confused about their reasoning.