I heard someone say one, "I not getting mediocre grades because I'm dumb; I'm just as smart as the smartest kids, but because I don't work hard enough ... Y'know, if I was really smart, I would work hard enough."

I've found this to be true in my college classes. For example, I took a class recently that was heavily related to previous research I did. The class was easy for me because I did previous work in that area.