I think we can agree that there is such as thing as a too-hard class, though? If I deliver a graduate level math lecture to a group of middle schoolers, they si

I think we can agree that there is such as thing as a too-hard class, though? If I deliver a graduate level math lecture to a group of middle schoolers, they simply won't gain anything from it. Teachers should aim for a certain optimal difficulty, such that most students are challenged but not lost. It sounds like this teacher was losing too many students, so not much learning was happening (at least relative to time invested).