Spare a thought for the relief teacher because very few of us full-timers do. Relief teachers are the white blood cells of the education system: they appear from nowhere to deal with sickness and then as quickly disappear again. Or you can think of them as wraith-like presences haunting the common room, vampires for a smile from the permanent staff. Which never comes.

Relief teachers are the untouchables. It is a caste thing. There is also superstition in operation. Like albatrosses relief teachers are somehow unlucky. Certainly you would never want to be employed at the whim of the daily organiser who is in turn subject to the vagaries of geography field trips and the flu season. Nor would any normal human being willingly submit to being lumbered routinely with six periods and a yard duty.

Relief teachers, however, are not normal people. Once they were artists and actors working for rent money. These days they are more likely to be old teachers who took early retirement and are now returned having discovered that their superannuation lump sum is not quite enough to fund the vaunted life of luxury. They have the look, these vets, of grunts presenting for their third tour of duty. They are survivors but may not be so lucky this time. Still they crack hardy. Call it gallows humour.

“Morning, workers.”

“Ho, it’s old Bill. How’s the golfing handicap? Run out of caviar and champers?”

“You fellas want to get some liniment for those shackle burns.”

But the faded wardrobe of the returned teacher — a sort of mustian fustian — proves traitor to the bonhomie. No one knows what becomes of relief teachers at 3.30pm. Nor is anyone about to ask.

Meanwhile, the day’s punishing schedule works its way through the relief teacher like a tapeworm.

“Er, substitute!” the children chorus at his every appearance.

Roughly translated this means “we have licence to shirk all work, listen to our iPods, text on our phones and give the poor sucker false names like Ben Dover and Phil McCracken.”

Forget Macbeth. This is stuff of true tragedy.