I've had this idea bumping around in my head for close to ten years now. Usually it would get paraded out when I heard about someone doing something stupid... the 'less/fewer' error was pointed out to me by my wife, Vicky, some time ago. Learning from the mistake (in the spirit of the comment itself) meant that I had to stop saying it, even though I still thought it was funny.
The blackboard and 'teacher's correction' idea hit me this morning and so I quickly put this together in photoshop. The idea, like me, is bettered by Vicky's input I think.
I doubt very much that the phrase was ever original to me but It's nice when an old idea finally comes to fruition and you don't see someone else who's made something of 'your' idea before you do.