Then there are the times when there is supply teacher drought - a period when five of your teaching staff are out on courses or with sickness, babies, scurvy, whatever. The headteacher is at a conference. On days like these you find three classes need covering and you and a pregnant deputy are the only ones available to do it.
Have you ever tried taking a literacy lesson with 100-plus children? Have you ever tried giving out 100-plus pencils or seating 100-plus children? This is what Piaget might have called the Zone of Proximal Ageing.
Of course your teaching skills are below par, because all reason has left the building - and you resort to shouting. It is the pair of you on the stage against an entire stadium full of children and you, the two performers, are becoming hotter and redder and more bad-tempered by the minute. And, it is wet play.
Then you pass a colleague who has spent the day in a blissful, serene classroom. She is cheerful, calm and restful and eager to share some anecdote.
You, on the other hand, have pencil sharpenings in your hair and are covered in sticky lolly wrappers, the result of trying to cajole or console a child who is: a - throwing a chair; b - shouting at you; c - eating a PE mat; or d - suffering from heat exhaustion from being sat by a Victorian radiator for six hours.
But it is also days like this that bring the whole school together.
Everyone pitches in.
The premises manager does turn the heat down. The kitchen staff slip you sly roast potatoes, exhuming a kind of wartime camaraderie. The classroom assistants support you by bringing you Pro-Plus and patting you encouragingly.
Generally the children are on form. But, it is hardly surprising if they get a bit scratchy. I am sure you would be too. Anyone who has been stuck on the Tube in peak summer while being desperate for a wee will understand.
Of course the next day, it is all in the past, all forgotten.
It is this period that is known as Amnesiac Educationalis. This is why I am writing this down now. Never forget what could happen.