I used that during my last semester and it was awesome. I set it so that my README.md would have my schedules, books to read, homework assignment, etc. I'd update it as the term would go on. I was able to keep track of all my projects at a single place even though they were unrelated, and I also took notes during the lectures. With how they display .md files, it's really easy and quick to take notes and still have something decent to read afterward.
If you use Emacs, you might want to check out Org Mode. It was basically designed to do that kind of stuff. Github also displays .org documents rather well.
I removed them from Github to leave place for the next semester, since we are limited in private repos and I didn't made them public since they also have answers to some homework.
Not only wiki since I also had a lot of source code throughout this. Also, since they added the feature to create files from Github, I used that a lot when I had downtime at my job so that I could work on school from Github. It could also be used in group projects I believe (unless we can't share those but I didn't notice if it's the case).
