I don't think this is generally true. I can certainly think of a few classes I passed while hardly showing up. But there are some reasons you might need to re

I don't think this is generally true. I can certainly think of a few classes I passed while hardly showing up. But there are some reasons you might need to regularly show up even without the case of a teacher who counts attendance (which the school might require of them if it's an intro sort of class). There might be unscheduled in-class quizzes (I think this is pretty rare). More commonly, though, the teacher may not have bothered to set up a course website, so without showing up you won't know what the homework is or when the tests are. The latter is something I remember quite a few cases of. Even if you don't show up, the course still takes up time in the sense that you can generally only register for so many classes at a time.