Here are sources for science fiction stories that I think would be suitable for middle school students. These are all freely available stories either out of copyright or made available by the author or publisher.
Most of these are very short stories or short stories. I think short stories are good because a few hard words won’t discourage a student–the stories read quickly. And if they don’t like one story they can try another.
At first I wasn’t optimistic that I would find stories available online as only stuff from the thirties or older is typically out of copyright. Here are a few good ones from Project Gutenberg:
Then I looked around more and found that many fairly recent stories are available online at the author’s site or some other apparently authorized site. This site and this site both link to a lot of good free sf. So now that it looks like there’s a lot to choose from I’ll make some suggestions.
Very short stories would be a good choice, this page links to a bunch of them, some only a page or so long:
Terry Bisson, “They’re Made Out of Meat”, (815 words)
Cory Doctorow, Printcrime, (688 words)
James Van Pelt, “Just Before Recess”, (782 words)
Nolan, William F, “Of Time and Texas”, (608 words)
Other great short stories:
Harlan Ellison, “Jeffty is Five”, (page 71, 8k words)
Kurt Vonnegut, “Harrison Bergeron”
Ray Bradbury, “A Sound of Thunder” (4k words)
Cory Doctorow, Anda’s game (10k words)
Neil Gaiman, A Study in Emerald (9 pages)
Isacc Asimov, The Feeling of Power (3k words)
Orson Scott Card, “Enderâ€™s Game” (15k words)
Larry Niven, “Neutron Star” (7k words)
This site also has section of ‘kids’ stories: http://bestsciencefictionstories.com/category/good-for-kids/
I found these Project Gutenberg stories first but though they are good stories I wouldn’t put them at the top of my list: