If one of our pupils were a self-harmer, we would try to protect them, as far as possible, from the worst they might inflict on themselves.

But when independent schools are approached by television companies, we seem to have an insatiable desire to put a match to our hard-won reputations. For 30 years now, independent schools have courted the one-eyed monster in the mistaken belief it will spawn positive publicity.

The casualties of this courtship are not hard to find. Consider, for instance, the headmaster of Radley expressing his uninhibited delight in 1980 at Margaret Thatcher’s unexpected elevation to power. Then, in 2003, the footage of Ampleforth sixth-formers hurrying out of mass for a quick fag in the woods or propping up the bar in the local pub, before falling off it.

Three years ago it was the turn of my own alma mater, Trinity College, Glenalmond, which was the focus of a series titled (surprise, surprise) Pride and Privilege. I’m sure the school’s warden felt he had “cracked it” as the camera lingered lovingly on the school’s beautiful Perthshire setting and elegant quadrangles and towers. You can imagine his sinking feeling a few minutes later when it emerged that the school’s first pupil on a 100 per cent bursary was being bullied because of it.

Then late last summer our school’s marketing manager was contacted by the independent television company Tiger Aspect, which told us they were making a documentary about independent schools and “the professions”.

Ignoring the rocky history, I thought to myself, “How bad could it be?” It was just one day’s filming, we could control their access and the producer seeemed nice. We swallowed our misgivings and said yes. Our first day of the new term began with a camera crew at the home of one new Year 7 pupil. The day continued with the crew in the cathedral for our first big assembly, then further interviews with a selection of genial parents, and culminated with an interview with me in the leafy glade beside our sixth-form centre.

By the time we got to that bit, I was feeling a good deal calmer. However, that calmness evaporated completely when the interview finally got under way. It wasn’t long before such words as “privilege”, “class”, “money” and, of course, “elitism” began to be bandied about. The temperature in the garden dropped to an almost perceptible chill, the questions got sharper, my answers became that little bit more defensive and I began to sweat and shift on the bench.

It was a pretty terrible hour - witnessed, I should add, by our now wanly smiling marketing manager and eight members of my media studies class, who were brought along to watch the TV crew at work. After the interview was finally over, one of the students came up to me and said, with a knowing smile, “You didn’t look very comfortable, sir.” I could only nod.

Life in a school is fantastically busy, so there were plenty of opportunities to forget the experience. However, by the time of transmission the only question I was asking myself was: just how humiliating would it turn out to be?

The programme began at 9pm on a Wednesday, and as the ever-affable presenter began to unleash the usual arguments about traditional inequalities in British society, the first casualty hoved into view: a fashion designer who squirmed on being confronted by the accusation that he was exploiting interns.

Then, exactly 32 minutes into the programme, with the literal tolling of Gloucester Cathedral’s bells, the section on the King’s School began ...

How bad was the damage? Did it lead to a call by the governors for my resignation for agreeing to it? Did it cause pupil numbers to haemorrhage? No. The twist in this tale is that the parents questioned came across as balanced and normal, the school as both picturesque and businesslike and this headteacher no more smug than he usually is. As a community, we breathed a collective sigh of relief.

But, for all that, a lesson or two had been learned. The few more customers phoning the next day for our prospectus were probably doing so out of curiosity rather than anything else.

The television company behind the film makes good, responsible programmes, but you can’t count on that now. The next time the phone rings and the television people ride into town, I might say yes. But I will, I think, on balance, say no. The anxiety has exacted a toll, and I have learned there are some risks you don’t have to take.

Alistair MacNaughton is head of the King’s School, Gloucester.