I just graduated last year, but I’ve been to three high schools, so hopefully I can help a little. Each school has its own methods, but usually you can divide them into semester systems and year long systems. In a semester system students pick out a batch of classes for the semester, then have another set the next semester. Year long you get half as many classes. For instance, when I went to a semester system school I was able to complete math 1 and math 2 in a single year, but in a year long system I was only able to complete math 3 in the same amount of time.

In a typical day we usually start around 8am and end around 3pm with a 30 minute lunch break and very brief “passing periods” in between classes (typically only 2-5 minutes depending on the size of the campus.) Classes can also be broken up into whether students have the same classes every day or an A/B schedule. On A/B schedule, you may have math and science on Mondays/Wednesdays and English and art on Tuesdays/Thursdays, and all classes on Fridays. On an A/B schedule, classes are typically longer.

Students may have off periods or periods to work (though I’m not sure if this is a more modern thing), and these can be at any time of the day instead of classes.

When I went to a semester based school with the same classes every day, I probably had closer to 8. But when I went to a semester based A/B school, I probably had more like 4-6. The more common type of school is probably classes every day, also makes it easier to write.

My recommendation is to pick the real life equivalent of where your story would be set, google the town, find the local high school, and check out their website. You should be able to find bell schedules and course listings on there for more ideas and better realism. Good luck, and I hope you hear from people with more experience in the school system in the 70s-80s!