Stick to short stories, exclusively or almost exclusively. Short stories have always been the medium which best captures SF, gets to the point the, "here's an idea, let's explore it some" nature of SF, while when things expand out to novel size it loses some of that (in spite of many great SF novels).
Plus, doing short stories makes it easier to keep people's attention, and less likely to lose people who've fallen a few chapters behind in the reading. Either you've read the story or you haven't. Changing stories day by day / week by week / whatever means you can get different styles in that appeal to different kids and break any monotony. It also gives you more flexibility to change your mind about course direction in the middle-if it seems like a good time to change direction, you don't have to finish slogging through the current novel first.
Also, you're not going to be able to cover the span of what you'd like to cover in one class, you'll have to leave things out. If you go with novels, you'll have to leave more things out.