If this is in school, they're classes, not lessons. Lessons are normally in books, or given privately by tutors, etc.

I thought they're interchangeable...

*they were

No. For instance, you could never say of a student that he “skipped lesson,” only “skipped class.” Also a class can be one hour long, but the “lesson” taught in it can take 15 minutes and the rest of the class is study time.

Good to know, thank you 👍

👍🏼