Do it like daylight savings time. Have everything start two hours later, all year. The issue for the kids is time related to sunrise, not time related to when the parents go to work.
Is that what it is? I thought it was total sleep duration. Do you have a link explaining more?
They're related. If you go to sleep at the same time (relative to actual-midnight) and get up an hour before school, you get more sleep when school starts later.
Or else what difference would any of it make? If all you did was start school at 9:30 instead of 7:30 and then kids used that to go to bed two hours later, nothing has changed. But when people go to sleep (and are inclined to wake up) has a lot to do with daylight.
That still doesn't seem all that helpful. If my high school had started at 10 instead of 8, I'd be out of class at 5:30 instead of 3:30, done with fencing practice at 8 instead of 6, etc. I'd finish the night's homework two hours later, and finishing that was already well past sundown even on the original schedule. A later start time wouldn't have been an opportunity to stay up later -- it would have been an obligation to stay up later.
The total lack of unscheduled time is an independent problem.