At home he is out of control but we contain him in school. He might see himself as a prince among men. He might like to turn up in school to enjoy an audience. His mates might wait for him to do something stupid and he will usually oblige. But really he is no more than a nagging unpleasantness in our day.

But last week a virus swept through the school and we had one of our legendary dysfunctional days. Lots of unknown supply teachers were drafted in and a sense of disorder was in the air.

Nicky had slipped through the net. With all the minor crisis management we missed the fact that Nicky was in a lesson - with a supply teacher. The lesson didn’t go well.

Nicky had a victim and an encouraging audience. There was “entertainment”.

It culminated in a confrontation. But then Nicky threatened to stab the supply teacher with a knife.

Now he would not have done it with anyone he knew, we would have defused the confrontation a lot earlier. But this was about having an intruder on your patch, with all normal rules suspended.

Of course, locking the door to detain Nicky once he started to swear was an error of judgment. We wouldn’t have done it. But the inescapable fact is that he threatened to stab another person.

The teacher sensibly unlocked the door and Nicky, realising he had gone too far, ran away. But it was too late - he had already released the demons.

His despairing mother was visited, reports were written, governors informed and a permanent exclusion arranged.

Nicky didn’t have a leg to stand on.

I am sure he did not grasp the consequences of what he had said. Even at the exclusion meeting he didn’t think we were serious. We would forgive him. He was only messing. It was only a supply teacher after all. But he hadn’t seen his snarl when he had pushed himself into the face of a young frightened man who had no idea who he was.

His head of year might have smiled and stared him out. But this was a young teacher who needed putting in his place. And so Nicky behaved in way that changed his future. We all felt implicated in the events.

We should have been more alert and prevented it from happening. But it was a difficult day and, when Nicky walked into that classroom and sized up a stranger, events had to run their course.

The teacher of course feels guilty, wondering if we will ever employ him again. But boys do not come with a rewind button. A moment of foolishness and Nicky fulfilled his destiny - a doomed youth.

Ian Roe is a pseudonym, he teaches in North Wales