And think of the girls too. You want a selection of books so that every student will be able to find something they will love. Most of the recommendations I'm seeing here would be for the male highschool student of 10-20 years ago, and the beloved wish fulfillment novel of a 14 year old male geek in 1985 won't translate that well to 2009. If you have Ender's Game or the Dark is Rising, make sure you have alternatives available.
Look to the World Fantasy Awards, Phillip K. Dick awards, Arthur C Clark awards, Hugo awards, Nebula awards of the last 5 years, 10 years tops. And the young adult variants too if need lighter works for the high school students. Get things more topical and from a modern pov that the students are more likely to relate too. Otherwise, getting something timeless will be pretty hard (eg. Bridge of Birds by Barry Hughart)