Same argument about distributions cuts both ways to me imo. Like you taught 400 students and you couldn't get a single one over the line? I think the immediate suspicion would fall on the common factor of the teacher. For a crazy event like that it seems much more likely that the instruction or assessment was flawed.

Mostly though I was just chuckling in my head about an old curmudgeonly professor of mine who was literally retiring at the end of that semester. We were all actually quite scared he would go out with a bang given his disdain for us. We weren't like 100% Fs or anything, but I think it was obvious we thought the material was pretty phooey and the lectures interminable.