lizwithhat: How to Serve Man. The Nine Billion Names of God. The Ones Who Walk Away From Omelas. Franchise. Marionettes, Inc. All ones I read at that age and can still remember by name, so they definitely made an impact

marnanel: P J Plauger's "Child of all ages" [themes thought particularly appropriate for that age-group]

ArdsleyWooster: Asimov's "Nightfall". Or there's a whole set in the anthology "21st Century Science Fiction" - some are suitable.

marnanel: http://www.multivax.com/last_question.html

[Non-public] "I liked a lot of Fred Pohl's 50s stuff at that age -- simple high-concept, funny stuff. Don't know if it'd appeal now."

miss_s_b recommended Ursula Le Guin and http://www.neilgaiman.com/works/Books/Smoke+%2526+Mirrors/

pseudomonas: my top-of-head thoughts were Lem, Egan, Asimov...