I taught in Portsmouth NH when the superintendent floated the idea of swapping start times elementary (k-5) early and 6-12 to start late. Many teacher friends who lived in town were excited as it meant their child could be dropped off at their elementary school and ride the bus to our middle school.

When the "shift" actually happened it turned out that due to bus costs ALL kids went later so that benefit was gone. The only one other nearby school that also made the shift Oyster River HS (in Durham NH). This meant for nearly every sport event requiring travel or not student athletes were dismissed early and missed their last class of the day (at least).

The so-called benefit of having sleep schedules match up with school times never happened as parents of lids those ages didn’t step up (shocker!) and attempt to get their kids asleep at the same times as prior.
If a kid normally stayed up until midnight now they stayed up until 1am for example, often much later. That and the fact that the overwhelming majority of kids that age had access to the internet 24/7 and had that access in their rooms meant that unless the parent physically removed the phone, computer, iWatch, etc from their "child" and shut down their home router their kid was awake late into the night. Even when the kid went to sleep "on time" it often did t matter when a less monitored/ dutiful child texted them at 2am and got them up for gaming!

I can attest that the number of kids who formerly showed up sleep deprived never changed that status. I’m convinced the whole shift only happened as a result of the superintendent's PhD thesis needed some data (I hope I’m wrong).

Bottom line, if parents aren’t willing to help their tween/teen make responsible decisions regarding rest and sleep needs, any shift in school start times is a joke.