In the US, it's common for schools to start at 8:00 or even 7:30. Recently there has been a push to start later to give kids more time to sleep. So pushing it back to say 8:30 or 9:00 (but not shortening the day, so basically everything just gets pushed back, and the kids stay up later, and need more time to sleep...).
The reason is purely practical- start times need to be staggered to allow schools to share buses, and having high school students get home first means they can look after their younger siblings until their parents get home from work.
Elementary school, 8am - 2pm
Middle school, 9am - 3 something (3:20pm, I think?)
High school, 7:40am - 2:55pm
It’s completely ridiculous, especially considering how sane many (most?) countries in the rest of the world have it.
Granted, even if they were on 2 income households in the 70's, it was also a time where 10YO's were trusted to get home or even walk home unattended, so it'd only be an issue for the youngest primary students. That may still be the case in some Asian countries today, but certainly not in the US.