Co-constructing does not mean that you abdicate responsibility, or don't provide a format for the class. It means you get feedback from the students on the material, and themes. If your students get so excited by a theme that it motivates them to explore read more widely and think more deeply that's great.
You can also have a core of short stories and novels that everyone will read and discuss, and let students develop their own personalized reading lists. At an in-between level, students can form groups to focus on a theme or an author
If you let students take a strong role in the class, make sure there is a solid place for kids who are new to science fiction or are shy about taking the lead. One thing you can do early in the class is to pick out some stories that you think are likely to be new to your class and make the discussion focus on the ideas, characters etc of the text. Don't relate it to Sci Fi/Fantasy as genres at that point. By the time you talk more about genres, students will have something to go on.
If you are willing to put in the time I would set up a wiki or a google group or other format for everyone to share ideas. (This would be in addition to traditional writing assignments)
By all means give kids resources for plunging in, but don't make the point an understanding of the history and range of the genres. [Personally I think either Fantasy or Science Fiction has sufficient scope for a course and wouldn't mix them.]
I read lots of science fiction in High School because I wanted to. I even read some good science fiction in classses ( Cat's Cradle, Fahrenheit 451, A Canticle for Liebowitz). I read it because it was fun, because it stimulated my imagination, not because it was someone's idea of canon. Don't include something you would hate to read. I know people's tastes vary, but I doubt my High School English teacher really liked Silas Marner.
/earth: file system full.