1. The professor is incompetent, and incapable of classroom management. 2. Every single student failed to learn the material due to their own fault.
Except that average IQ drops quickly when in a large group. Sadly, if there are 3-4 real troublemakers in the group and enough "indifferent" students, it's probably enough to make at least 75% of them behave like retards, and that makes it a heck of a lot harder for the other 25% to actually do the learning. So it's very possible that in such an environment, no student was able to learn the material, due to the behavior of the group as a whole.