Once, when I was a deputy head, I was in the staffroom at break time, joking with colleagues, when I realised that the break was well and truly over and all the children would now be in the building.

I looked around, and began to say something friendly and encouraging like:

“Well, folks, I think it’s time we went to class.”

However, I got only as far as, “Well...” when the door burst open and the headteacher came in. He looked around, jerked his thumb towards the door and made that quick whistle between the bottom lip and the top teeth that means, “Out!”

It was a good lesson. In fact it was several lessons. I learned that, as deputy, I should not become too comfortably ensconced not only in the staffroom but in its culture - and that, when I became a head, I would be well advised to cut off from it completely.

I learned that for one member of staff to be late to class is no trifling affair, but something that exposes children to physical danger, including increased opportunities for bullying.

And lastly, I learned that while gentle admonition is fine, sometimes you need a bit more, in which case the thumb and the whistle come in handy.

Except that I could never manage the whistle, which rather spoiled the effect. Recalling this, I began to think of other examples of lessons learned in the same quick and salutary way.

There was that sports day, for example. I was a new head by then, and I was determined it was going to be a good fun event, with lots of parents.

The deputy head got busy with charts and lists, as they do. The caretaker covered the field in a bewildering maze of ropes and posts. Teachers gathered their teams around them during the morning and gave them pep talks.

Then out we all went, and there began a magnificent and brilliantly organised pageant of physical endeavour, while I walked self-importantly around in a straw hat nodding at the parents.

Then it rained. Actually it poured - suddenly, and comprehensively. And everyone - parents, children, teachers - just raced for indoors. In seconds the field was deserted, posts and ropes scattered, bits of equipment lying about, while the interior of the building was a jammed and yelling mass of very wet people.

I stood there entirely helpless, mouth opening and closing. Short of firing a revolver over the heads of the crowd, like First Officer Murdoch on The Titanic, I could not think of anything to do except gradually calm things down with the help of colleagues.

It was the most disorganised, unseemly few minutes I’ve ever seen in a primary school, and I was in charge of it. And all because I failed to ask (or even think) at the planning stage, “What do we do if ...?”

From that moment, my deputy, a hugely practical person, had the specific job of asking the “What do we do if ...?” questions in all planning sessions. (You can still get caught out, of course, but at least the mindset is in place.) Did you have some key learning events in your career? Of course you did. It would be good to hear about them. I asked my friend Bob, a long-serving head, about his.

“One of our teachers had been having a good parents’ evening, friendly and relaxed,” he said. ”

“Then the last couple she saw verbally savaged her for no apparent reason.

We talked about it and realised together that when parents are angry, it is often because they are being defensive about their own belief that they are not coping. And that sometimes, if they have issues with each other, they’ll deal with them by both attacking the teacher.”

That being so, he realised, there was no point, and certainly no advantage to the child, in getting into confrontation, and that building bridges with parents was not necessarily going to be as easy as the textbooks can imply.

What can really bring you up short, though, is to look back at your career and realise that there were many events from which you failed to learn anything at all.

When I was well into my headship, I told a veteran head, with whom I had once worked, that I was thinking of applying for a new job. In a second headship, I suggested, you can look back at the mistakes you made the first time round.

“And make them all over again,” he boomed cheerfully, smiting me on the shoulder.

Gerald Haigh is a former headteacher