It's worth noting that this IS only obtainable in the school and appears to be missable but is NOT tied to your first visit there, so don't waste time trying to get them to bite you on your first trip. They WILL do it, but not NEARLY as often as on your second trip.
Also worth noting is that it doesn't have to be your character they bite. Biting your "buddy" will also work, and the number is cumulative. In my case it was me once and Butters twice, and it popped.
That being said, there are spoilers below. If you don't want to know, just trust me you can do it easier on your second trip and leave now...
Okay, now that that's out of the way, the second time you go into the school in the big Elves vs. Humans war, the ginger hall monitors will have turned into Nazi zombies. They still have several different attacks, but the one you want gives a bit flash 'Nazi Zombie Bite' across the screen before they approach. If you see this, don't block.