The theory that it's possible to avoid being bollocked for misbehaviour if it's so blatant that the teacher will be unable to believe it actually happened, or incapable of responding in a dignified and appropriate manner. For instance, a twenty minute tirade about "stringy chicken dicks" and "this woman, right, gets shagged by a horse" in response to reading Animal Farm (qv) will generally result in a feeble "that's enough now, Paul" Immunity in excess does not extend to those laughing, however, who will be punished with misdirected severity.