Most successful Wikipedia assignments — at least, ones that include a major research and writing component — depend crucially on the subject-area expertise of the instructor, and the more familiar you become with Wikipedia's coverage of the topics your students will be editing, the better things will go.
In short, you should be able to point your students to articles where they will easily be able to make improvements, and recognize which topics are beyond their ken — or too contested for newcomers to jump in productively. The more you get involved with content discussions yourself, the quicker you will develop that kind of knowledge.