My brother is a physics professor and teaches a General Physics or Physics 101 course in a big lecture hall. He says that the back 1/3 of the room is tuned out, on their phones, talking to each other, with some kids even sitting in the back behind the chairs having a sort of Pow Wow while he's teaching. He had a different professor come in and sit in the back to see what would happen (everyone knew this was a professor), and it had no effect. They just didn't care.

He said he gave half the class D's or F's because they didn't even do the work.