It's a bit depressing to me that we've known this for at least twenty years, and possibly more and it's still a problem.
A major concern has been busing. Even in normal times, districts use the same buses and drivers for students of all ages. They stagger start times to do that, with high schoolers arriving and leaving school earliest in the day. The idea is that they can handle being alone in the dark at a bus stop more readily than smaller children, and it also lets them get home first to help take care of younger siblings after school.
If high schools started as late as middle and elementary schools, that would likely mean strain on transportation resources. O'Connell said Nashville's limited mass transit compounds the problem.
"That is one of the biggest issues to resolve," he said.
This is basically it, school systems not wanting to buy the extra buses or hire the extra drivers they'd need.
Unfortunately I don't see this ever being solved without a major cultural/financial shift in the USA towards properly funding education. Too much financial pressure to have fewer buses and fewer drivers. If my high school and middle school had started at the same time as the elementary, that'd be like 14 new buses alone at $60k-$110k a pop, not including driver wages and the diesel for each one...and we had more than one high school and middle school in our district. So it'd be more like 50 new buses, just to start HS and middle school at the same time as elementary. The cost would eat smaller districts alive. It'd be several million just to procure the buses new.