Having a camera team in school is a risk. But when we were approached by a television company working with Channel 4 last autumn I felt we were good enough to withstand that, and the kids would come over well, so we agreed.

They wanted to film a young man who had left school after deciding not to go into the sixth form. They wanted to see if we could hook him back into education. Filming started in January. Some sixth-formers were given cameras to make video diaries. We were to help with the production and be involved with the editing - this was a great chance for kids who were interested in getting into the media.

Then the production team introduced our new pupil. We had expected some trendy, good-looking guy who would be a bit of a Big Brother star. But Howard Simmons, as he was then, was an ordinary, nerdy sort of chap. He said he’d been bullied at school in Cardiff and had ended up being taught at home. He was quiet and shy - and he did look 17. He had a brace on his teeth and spotty skin.

But he was not what he seemed. The brace was part of his disguise and the spots were make-up. It was well done, though. I taught him general studies for 10 weeks, and never guessed. Our sixth-formers were with him every day. If anybody was to suss him out, they would have. We felt sorry for him, and the sixth-formers took him to their hearts. They showed him around, made sure he knew where the dining room and common rooms were. The first weekend they invited him out to a club.

Filming went well. A week before the end of term, the producers invited me and the head of sixth form to lunch. We thought it was just a thank you. But towards the end of the main course the producer told us the truth - Howard wasn’t 17, he was 30. And his name wasn’t Howard. He was a TV producer called Simon Simove. We froze. We spent the next two hours asking why. We felt betrayed.

They said a real 17-year-old might not have worked. We told them the sixth form would be devastated. He’d shared a 17th birthday party with one of the girls - that’s how close they were. I kept feeling for her, thinking: “Oh my God, she’s going to look back and think the whole thing’s a sham.”

They wanted us to break the news to the pupils, and they would film us doing it. I said: “No way - they’ll be devastated.” They replied: “No. They’ll be shocked, but they’ll laugh.”

Next morning, we had a meeting with Channel 4 - five suits, including a lawyer, arrived at 9am. I was so upset I had to walk out. The chair of governors followed me out and asked: “Do you want all these people in the school?” I said no - she asked them to leave.

I decided to call the sixth form together. I’d asked Howard and the producers to come in, in case the students wanted to speak to them. They owed us that much. We brought them in and they sat there, white, while the sixth form told them exactly how they felt. One girl burst into tears. Howard apologised and he cried too. He was still in role, still calling me Miss - it was weird. It ended when one girl stood up and said: “We’ve heard enough. We don’t hate you as a person - we hate what you’ve done. Please leave our school.” It was powerful. They filed out in silence.

The story broke, and for the last week of term the media seemed to be everywhere. Reporters were trying to get in, and knocking on governors’ and teachers’ doors, stopping kids in the street and offering them money for photographs. To get rid of them I held a press conference, and from there it went on to television and the front pages. The quote of the week came from my daughter: “Headteacher of Warminster school knocks Kylie off the front page of the Sun.”

Some of the people who’d been close to Howard were still upset when they got back to school, but we kept talking it through. We feel sad. This was a lost opportunity - not just for Kingdown but for comprehensive schools all over the UK which are sometimes shown in such a bad light. I hope the students don’t get too cynical about it. We can laugh about it now. I’m sure their graduation speeches are going to reflect it fairly well.

Shelagh Brown is head of Kingdown community school, Warminster, Wiltshire.