One of my professors, who happens to be among the top engineering faculty we have said something interesting when I was talking with him in office hours... He used to measure his performance by the grades his students got, then realized that if the student is motivated, they'll learn and come to class and pay attention. If they don't want to pay attention, then they're not self-motivated enough to make it anyway. If they fail, that's because they didn't take full advantage of the class. Let it be said he teaches upper-division courses, and really, if you make it that far you should be motivated enough to pay attention in class instead of facebooking anyway.