Is YA Lit "Literature"?

Some schools teach modern YA lit alongside the Great Gatsby and Catcher in the Rye; some teachers wouldn’t be caught dead saying the words “Young Adult.” You can now get an MFA in YA writing...unless you go to one of the top MFA programs in the country, where they say “that’s not real writing.” But what is “real” writing? What is literature, and what makes a book “literary”? What does it mean that half the audience for YA is over 18? And what are you supposed to say to your teachers/ parents/friends when they tell you drop the kid stuff and try a “grown-up” book? (Other than “#$&@ you!”)