Let me tell you a story that will explain why a certain teacher is no longer on the educational front line.

Once upon a time, a student called “Harry”, the wannabe star of the special needs class, was bellowing in the corridor to a group of girls - directly under the sign that read: “Silence: examination in progress.”

The teacher on duty knew an A-level exam was taking place, so approached Harry and hissed very quietly, “Can’t you read?”

“No!” declared Harry proudly.

The five-minute warning bell for the next lesson had gone four minutes before. But, in the circumstances, every second of bellowing was critical. These circumstances were:

1. The teacher’s daughter was one of the pupils just inside the classroom door taking her A-level. She needed an A to meet her university entrance requirement, so her future hinged on it.

2. It was the first year of this A-level course, which the teacher had set up and retrained for at MA level. There had been major investment from the school, huge commitment from the teacher, and the class now needed to repay both with good grades.

3. The teacher had shown impatience with county directives, increasing the pressure if the group failed.

4. He was on his own in a corridor with a bellowing idiot, trying to protect everything he held dear, with no real powers - not even the ability to yell or sound fierce, as this would disturb those in the exam room.

The multiple ear-studded, baseball-capped, gum-chewing student now exchanged noisy “see you after the next lesson” hugs with all three of the girls.

It didn’t matter what time Harry got to his lesson: its content could be absorbed (even by him) in about 15 seconds, and he was getting his main subject - social education, defying authority - in the corridor.

The teacher took a deep breath, and said quietly: “There’s an exam going on just behind you, Harry. Let’s move on to our lessons, shall we?” Then he laid a hand gently on the boy’s shoulder to steer him softly in the direction of the exit.

It was the lightest of touches, but, of course, the 50-year-old professional had just handed the entire situation to the 14-year-old.

“Fuck off,” said the boy, in such a studied act of defiance, the teacher could almost see the speech marks hanging in the air around the words.

“What did you say to me?”

”`Fuck off.’”

Now the teacher shouted, his fury increased by knowing his volume would do the very thing he was trying to avoid: disturb the exam. The boy then went through another round of “see-you-in-40-minutes hugs” with the girls, while the teacher, disarmed now of even his moral superiority, glared in helpless rage. The boy smirked and exited, almost blowing a kiss.

His mind clouded, the teacher went into the exam room to assess the damage. He was a disturbing presence, replaying the scene over and over, adding to the tension between himself and his exam charges.

He shouldn’t be there anyway - in loco parentis and also a parent, supervising an exam and that exam’s teacher. He looked in vain for the poise he lost in the corridor. He stared helplessly at the legitimate supervisor (whose daughter was also present), who shook her head.

The teacher left and tried to impose order in retrospect by writing up an incident report for the head. As he wrote, his confidence grew: the fact he had tried to protect an A-level exam from disturbance, and the fact the student had told him to “fuck off”, then deliberately repeated it, gave the teacher hope that his lapse in professionalism would not lead him to be the one landed in trouble.

Think again, Sir. At the end of the school day, it was the teacher on the carpet.

“Harry says you touched him.”

“Yes, but only as a soundless . I was protecting . ”.

“So you did touch him?” The head almost seemed relieved: now he had clear procedures to follow.

So what happened to the characters in this story?

For disturbing an A-level class’s life chances, the pupil got a day’s holiday (at the insistence of his year head) and a year to gloat over the diminished authority of the teacher.

The pupil’s father got to make a call asking for the teacher to be “struck off” for striking his lovely boy.

The teacher got the best career advice he’d had in 27 years of teaching and running departments along the front line: to fuck off.

And two years later, hurt and frustrated, that’s exactly what he did.

That teacher, of course, was me.

• Gareth Calway, A-level English examiner and NATE consultant.