Gay Schoolboy's Grade

Alright, listen up, smarty pants! This "joke" is playing on two things. First, the obvious: it's a play on words, using "D+" which sounds like "D's plus". Now, for the second thing: the joke implies that gay schoolboys like D's, as in, um... well, you know. Look, I ain't gonna spell it out for you. You are a human of the 21st century, not a bear of the ice age.