My kids are lucky in that we're close enough to school that they can leave at around 8:15. When I was in high school I had to get the bus at 7:30, which was a real struggle. The idea of starting school at 6:15 is just pure insanity.
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.
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.
When I had sports I would usually get home from practice at around 6-630 at night. Ideal time management would mean I'd eat and do an hour or two of homework, but usually I'd stay up until ~2AM using dialup internet to MUD with friends. I was exhausted! Playing MUD did help lead me to my career though, so I suppose it wasn't all bad.