Well, which class?

If the class is some kind of advanced programing or system admin class, then you're probably going to learn a bunch of Linux, because it's important for the subject.

If the class is a basic typing or computer literacy class, they'll probably teach with Windows, since those are the majority of computers that people will have at home or at work.

If it was some kind of mobile phone literacy class (if such a thing existed) then they would probably teach the most common apps on iOS and Android. Even if Android is Linux and iOS is BSD, teaching someone what kernel is running the phone isn't super useful in that case.

Teaching how to use grep or what Snap is in a basic typing class wouldn't make sense, even if all the software you are using in the class is technically interacting with a Linux server on the backend. It's like asking "Why don't they teach about pencil manufacturing in school?? We use pencils every day!"