text: You can set up an online forum on any topic you want easily enough.  You could just declare that the rules are you can only discuss the particular topic, and only in question and answer format.  If you're using it for a particular group (i.e. your class), you could just keep track of points on a whiteboard or such.  It probably won't be quite as addicting as Answers, though, because part of what's so great about Answers is only can discuss whatever topic you're interested in.  If you're trying to get kids interested in something their not, the Q and A format might be more engaging that straight lecture, but they won't find it as much fun as posting silly questions and getting silly answers.