The darkest hour is just before dawn, according to the old wisdom. In the case of teachers it is that interesting fifteen minutes before the first locker bell. (Definition, locker bell: the alarm signalling the instant filling of the corridors like a surge in a stormwater drain.) It is not the awful anticipation of dealing with that one student who has been fated to make your life difficult in the extreme (“I said SIT DOWN, Tarquin.”) — although that is terrible in itself; it is, rather, the neurosis induced by the posting of the day’s extra classes.

Now, somewhere in the shady history of industrial relations between management and labour, those useless bums, the union hierarchy, sold us poor workers down the river. And so it happened that whenever a fellow teacher is away, those in attendance are made to cover the absence. Thus do normally sluggish chalkies acquire a turn of speed when the daily organiser — our very own Nemesis — comes seeking for someone to fill that gap in the time-table. A carrier of the plague could not be less popular. Sadly, after long experience, he knows exactly where to seek out the reluctant.

“I know you’re in the stationery cupboard, Trevor. So you might as well come out.

And so you trudge towards your doom.

“Sub,” the kid keeping nit announces well before your arrival. Sub, of course, is substitute. It is synonymous, in the teenage mind, with that other word — bludge.

The worse of it is that often you will be covering for the staff malingerer who seems to be absent more often than he or she is in attendance. It explains some curious classroom practices.

“Mr X just lets draw in our diaries.” Or “Miss Y lets us listen to our iPods.” All this chorused as if reciting the Declaration of Rights.

Normally the absentee is required to leave work for the substitute teacher. The anal retentive supply you not only with comprehensive tasks sufficient for an entire semester but also with class lists and photos and a potted history of each student’s psychology and academic performance. Not, however, the staff malingerer.

“They should have work to go on with,” the instructions read. They never have.

For 50 minutes you do the educational equivalent of a tarantella and by period’s end you’re knackered. No wonder those who escape an extra behave like fortunates who have dodged a bullet. Their time will come. Like the turning of fortune’s wheel. What goes around…