What is the difference in usage/meaning between クラス and 学級? Why did I never learn the latter in language classes?
クラス at school can refer to:
- a group of students who learn together (このクラスには生徒が35人います。)
- a lesson, a lecture (5分後に数学のクラスが始まります)
In kanji, the former is 学級, and the latter is 授業/講義.
学級 and クラス (in the first meaning) are basically interchangeable, but 学級 is typically used in elementary and middle schools. For some reasons, people start to prefer クラス maybe after entering high school. (Does this explain why you didn't learn 学級?)
学級 - Wikipedia
And there are many set phrases where only one of the two can be used. For example, クラス替え is usually not called 学級替え, and 学級閉鎖 (due to flu outbreak, etc.) is far more common than クラス閉鎖.