I didn't even read The Scarlet Letter in class (later came to appreciate, if not love it), but I enjoyed Animal Farm a great deal, probably because I was and am obsessed with Soviet history (I was a weird kid).
I read Vonnegut on my own and didn't discover Joseph Heller until my 20s, but I concur with everyone else that they would be good choices. I'm a big Twain fan, too. Ditto for Melville, though I don't know if high school is the best age for him.
Other stuff:
Dave, I totally agree re. Saunders as an essayist - he's like the Bizarro-world David Foster Wallace, in that his fiction is so amazing but his essays merely readable.
Eric Luxury, don't forget all of that New Yorker work that Tomine is doing. Also, Clowes has that weekly strip in the NY Times Magazine. He's not really that much less productive than in the early days of Eightball - didn't that only come out a couple of times a year? I also think that Jason has come onto the scene at a time when alt-cartooning has become more mainstream, which paradoxically makes each new cartoonist less noticeable, if that makes sense. But I can't blame cartoonists for bailing on a medium that pays so horribly (Pantheon editions of graphic novels notwithstanding). If I was independently wealthy and not indentured to the elaborate scam that is the Tisch School of the Arts, I'd write weird experimental plays for the rest of my life, but as it is I'll probably have to jump ship at the first opportunity.