Actually from Megan McArdle of the Atlantic Monthly and for Bloggers, but I though it was all relevant for classroom discussions, especially this section:
Treat stupid questions as if they were serious. Has anyone ever asked you "What are YOU looking at?" in a bar or other public place? That person was, of course, looking to do the Monkey Dance, work his way up to chest-poking or hat-knocking-off, and perhaps eventually a sucker punch. Blogging works much the same way--monkey dancing is the dominant mode of commenting. But on the other hand, if you actually try to answer that drunken idiot's question--act as though you don't detect the challenge, point at something and call the questioner's attention to it--you can get inside his OODA loop (unexplained allusion! Look it up, people!) and diffuse a bad situation. In blogging, treating a stupid question seriously can accomplish one of two goals: it can highlight the stupidity (because the answer is stupid, or obvious), or it might (and sometimes has, for me) elevate the discourse by pulling your interlocutor off of his stupid position and into a real discussion. Either way, you win, because you aren't the one being a jerk.