My kids' school district (Newton) shifted the High School start time to 9:00 from 7:40 this year, a huge change. There do seem to be some benefits to starting later, but there are clear trade-offs.

It really messes with after-school activities, particularly sports because you have to coordinate with other schools. Our HS end time is now 3:45, so by the time kids can get on a bus to travel to an away game it's 4:00pm. By the time you arrive at another high school, game time may have to be 5:00pm. Now you have to worry about commuter traffic, sunset times, and pushing dinnertime to be quite late. Not to mention students at the other school have been out since 2:30pm or whatever and have to wait around for the Newton team to arrive a couple of hours later.

Sure, the sports team can leave school early. But if one outcome of a later start time is having athletes miss school, then that's not great.

Another issue: during the winter, it's starting to get dark while kids are going home from school. That's a safety issue, and it also just sucks to have zero daylight after school.