At 3.30pm on the Friday before half-term, Ms Johns appears at my door. I don't need to ask what the problem is or why it has been left until now to discuss it. The only question that isn't yet answered is: "Who is to blame this time?"
The moment when a class begins to slip beyond your control can be hard to define, either at the time or afterwards. For most teachers it is a gut-wrenching and life-defining experience, when your capacity - or lack of it - to control a group of teenagers affects how you see yourself, not just as a teacher but as a human being.
You question your value. The prospect of taking that class again stops you sleeping, spoils the weekend, wipes out the hours before you see them. You plan and you strategise. You consider asking for help but don't - or you ask everyone for help and become overwhelmed by well-meaning advice. But in the classroom it always boils down to you and them and a battle of nerves: your fracturing self-confidence against their bolshie, mouthy rebelliousness.
Every failing teacher talks in generalities. It's "the whole class", "all the boys", "every student" who has played up. None of them has manners, understands respect or accepts authority. The problem started at primary school, at birth, before birth. It's parents who are at fault, or society, or a world gone mad. The school is a thin blue line in a world where young people accept no discipline. There is no proper training, no induction, no professional development. The problem is the school, where there is no order, no discipline and children run wild.
The issue here is of something fundamental that can't be taught to the young teacher - or, indeed, to the older one. It's about having warmth for the young people in front of you that has nothing to do with sentiment and everything to do with belief in people in general - and in young people in particular.
It's about knowing that education is worthwhile and that you won't be deflected whatever the shenanigans in the room. It's about recognising your own worth as a teacher and theirs as pupils. It's about the hard slog of lesson preparation and marking and endlessly giving your best when very little is given in return. Until one day the tide changes, as it almost always does, and you find the soul of the class and, perhaps, they find a little of yours.
So, Ms Johns, my heart goes out to you when you tell me I don't understand; when you tell me you have been let down by the system; when you tell me the pupils are unteachable, that they are animals; when you see them as a whole, not as individuals to be valued. But it is you who keeps me awake at night.
The writer is a headteacher in the East of England. To tell us what keeps you awake at night, email firstname.lastname@example.org.