I had a calculus class at uni that started so easy that no-one was showing up after week one, and the module was 90% exam, 10% coursework, so people weren't bothering to do the assignments either. The people not showing up didn't seem to realise that the class ramped up to multivariable calculus by the end of term (Britain doesn't really separate calculus into numbered stages in the same way the US does), and the final revision lecture before the exam was completely full and the room had an air of absolute dread now that everyone knew what they'd been missing.
I had a class that was so easy that most students didn't bother to show up for lectures. Near the end of the semester the profesor got angry that nobody came to his lecture and decided to retroactively grade attendance with 70% of the grade. Most students flunked. I was fortunate that his class was in between two others I was taking, so I attended about half of the lectures and passed it with a grade that corresponds to the American C+/B-
Not a very high pass rate on that class.