For some unknown reason I have some Internet thing going with the TES, so they send me an email weekly with various links. this week's includes inspectors who fall asleep in your lesson. Shame.
My first thought would be to take out a mobile phone and film them Zzzzing and then wander casually over to a piano and play loudly. One fff chord would suffice. Every room I teach in has at least one piano... Snigger.