I read Farhenheit 451 in eigth grade and LOVED it. (I’m in tenth grade now.) ALthough the book itself is rather short, there is a LOT of opportunity in there for class discussion.
Now that I’ve had some time to mull over this, I recommend “The Things They Carried” by Tim O’Brien
Ender’s Game, by Orson Scott Card. Take it from an actual teenager, we (the males at least) love this book. Simple enough writing style, and a plot that’s both exciting and thought-provoking. Don’t try the sequels, though, the philosophy gets tougher and the plot drier.
I forgot one other book that I loved in high school (when I was a freshman)–The Hitchhikers Guide to the Galaxy by Douglas Adams. Loved it, kicking myself for leaving it off of my prior suggestion list.
I would have to agree with the Evil one. Although it might be kind of complicated, The Things They Carried is a great book. Tim O’Brien is a brilliant author and this book would definitely suit a short story class.
A word of caution: books such this are only good for classes that discuss them (i.e., the teacher should not be lecturing.) Other than that, The Things They Carried would be my first choice.
Martyn Pig by Kevin Brooks. A very good book. One of my favorites now, actually.
The title character kills his dad ‘accidentally’ and instead of telling the police he enlists the aid of a friend in disposing with the body. Stuff happens. Jealous BF of friend gets involved. Plot thickens. Words are said. People are killed. Hallucinations are had. Very good. And only mild language.
It may seem too juvenile, but what about The Phantom Tollbooth by Norton Juster? It’s certainly good for improving one’s vocabulary.
Also, The Red Badge of Courage is worth considering.
John Marsden’s Tomorrow When The War Began series is very very popular in Australia. Heavy themes like war and separation from family and killing and sex and all those fine things 
I know it’s available in the US but I’m uncertain how available. I don’t think the series is outstandingly good myself but it is outstandingly popular with reluctant readers.