I'm a sucker for Drupal. I've followed this software for about 5-6 years now, sometimes straying to others but always coming back. While the technical end of it frustrates me (I'm a dangerous coder), I'm always drawn by that power to do just about anything. Just getting my head wrapped around some of the basic concepts takes time, but every time I play with it, I get more out of it. With the holiday break coming up, I might have some time to tinker.
I've already set my class up as an Organic Group and enrolled my students (who seemed a bit baffled by the invite email). Now I'm exploring options for calendaring with the good folks over at the Drupal in Education Group on drupal.org.