Sci Fi book choices for 12th grade

About a year ago, I sent a request out for some Sci Fi books for 12th graders to read as part of their unit in short stories.  Here's a link to that post on a different blog which is worth taking a look at just for the few titles I collected in the comments. I've copied the post below:

J’s 12th grade co-teach class is going to do a unit on science fiction short stories and wanted a list of a few good ones. So I’m posting here to get some titles.
I suggested Arthur C. Clarke’s Nine Billion Names of God and Asimov’s Nightfall. I also gave some authors: Zelasny, Heinlein, L. Niven, a couple others.
Nightfall is hardly short at 300+ pages but it fits the genre in other ways.
Any other suggestions from folks?

It's that time of year again, so I've got a couple of students asking me for more suggestions. I emailed this same list to them and I'll again ask for suggestions this year so I can have a wider list every time I send it out.

Any more suggestions?  -- I guess it doesn't have to be a short story, but that is the unit even though the focus for their assignment is on the Sci Fi part, not necessarily the short story part.