Ender's game is a must -- but I would add as supplemental reading Ender's Shadow. It's a super interesting example of parrallelism in story telling, and provides another view into things.
Of the available works of CS Lewis, I would of course recommend his Fantasy stuff, in particular the Lion, the Witch and the Wardrobe. What I've only discovered very recently, though, is his Sci Fi works. While still as religiously driven as the Fantasy works, I think that it was a different take on it, and provided some good discussion points. Also, it's something that he's not associated with, so might be interesting to see the different angle.
Dune is another amazing book, and I think it's uncommon in the Sci Fi genre as a 'world building' book. The sort of stuff that I more regularly associate with Dragonlance, Tolkien or Anne McCaffrey. While I really enjoyed the book (I read this in Grade 9 Literature) I think that it might be too long for the average class.
I would definitely recommend a Harry Potter novel. It's most certainly fantasy, and has enough worth commenting on; although I might offer it as a suggestion for those who are intimidated by the larger books. As an alternative, the Tales of Beedle the Bard is something that is short, and has easy morals.
Other than that, I think you need a heinlen, and an aasimov, and possibly a sagan (go short stories for all three if you can).
It's required that you include tolkien, but I'd make it either the Hobbit, or just a smattering of selected readings. I think it can be assumed that people will be familiar with the world, if not from the books than from the movies.
I'd also consider something by Margaret Atwood, to demonstrate that Science Fiction can be Science Fiction even if the author refuses to admit it.
The Chrysalids is a commonly selected novel for lower grades, but be prepared for people to get really upset on a religious level. Also, the ending is brutal.
Finally, the first book in the hitchikers guide is a great example of comedic sci-fi.
As for the original post, I really don't think that Bradbury fits here. Sure, it will win you points with anyone who isn't a fan of the class, as it's considered classic lit. But, I don't think it's so remarkable that it would beat out the other stuff you mentioned. Orwell would be a good substitute though.