Tangentially related, there are also studies showing that schools that force kids to get up at 6:30 also produce lower performing students because kids stay up late everywhere and then never get enough sleep.
If you want to keep the public school work week, it would make significantly more sense being 9-5 or even 10-6 Monday to Thursday rather than 8-3 Monday to Friday.
We aren't going home to work the farm, and we aren't setting insanely early sleep schedules to get up with the sun to till the fields. These ancient time systems cause legitimate harm for the sake of posterity.
Note that those studies apply to High Schools. Young kids are notoriously early risers.
Here in Ottawa, Canada, they've stumbled on a good solution. To save money on busing, they've staggered the school start times to allow bus sharing. In my area, that means the buses drop elementary kids off for an 8 o'clock start and then pick up high school kids for a 9 o'clock start.
As far as your suggestion of running four 8 hour school days instead of five 6 hour days, I certainly hope you're talking about high school. 6 hours is already too long for my 6 year old, I can't imagine 8 hours.
The justification I was given is that high school students need to be home first, as they might be responsible for watching their younger siblings once middle/elementary school lets out.
If you want to keep the public school work week, it would make significantly more sense being 9-5 or even 10-6 Monday to Thursday rather than 8-3 Monday to Friday.
We aren't going home to work the farm, and we aren't setting insanely early sleep schedules to get up with the sun to till the fields. These ancient time systems cause legitimate harm for the sake of posterity.