HP prank, probably impractical (you wanna buy a bunny?)

[Necratoid]

Technically it would only take a very, very, very bored first year accidently magicing Binns with raw willpower fueled by extreme bordom of the borred to do that.  You can even have the topic be completely random and change every class... unless they have someone in power watching and then its normal.  Basically, Binns in now randomly switching with his dimensional equivalent and no one notices that part... until the tests comes up and its on things they haven't been lectured on or not.