It would also require a much larger gym and much more equipment. Same for all other subjects: many more musical instruments for music class, etc.
Many school districts around the country are far too broken to worry about start times. The republicans have done a very good job of starving education for money over the last 20 years, and rampant corruption within the teachers unions and school/district administrations doesn’t help the situation.
I got to go to a fancy school where there were like 12 kids in the class. It was amazing, I learned so much during my time there. Conversely, I went to some pretty shoddy city schools as well and the teacher could barely get the students to just be quiet.
I will say in yet another school (I moved A LOT) for my final semester of high school I went to an alternative school where you did all your coursework on the computers. You could work as fast or as slow as you wanted. It didn't take very many teachers to run that at all. The computer took care of most of it, including hints and letting you rewatch how to solve the problem. I loved this-- school was always kind of boring and I was always one of the first to finish and just end up reading a book...but I was able to finish up my whole class extremely quickly (I think a month or so, it's been a few years).
Could you elaborate on this? What is the reason? Saying it's "more economically efficient" than grouping class types is something many of my teachers would disagree with, as they have lives they would love to get to outside of school (e.g. if a PE teacher gets out early in the morning, they can go to their second job or see their own kids) and complain about having to stay at school because there isn't enough time between similar classes that are spread across different times of day.