The children of Westmoor junior school were enjoying their morning playtime when one of them spotted him. "Miss, that's the man who took our keyboards," a child whispered urgently, tugging the arm of the headteacher who, conveniently, happened to be outside.
Within seconds, news that "the man" was back in school passed round the 300 children like an electric current. They gathered in a semi-circle, crushed around the 6ft stranger who two days earlier had interrupted their morning music lesson to collect two faulty electronic keyboards. Half an hour later the official repairer had arrived, exposing the first man as a fraud.
A police officer, also conveniently at the school in Dewsbury, West Yorkshire, arrived wearing a bullet-proof vest. He arrested the suspect in handcuffs before hauling him into school to face the four classes who had seen him earlier.
Only then did PC Derek Allan, a community service officer with West Yorkshire Police, let them into the secret: the "crime" was a set-up.
Although a handful of children had suspected something, the majority believed the entire scenario.
The idea is to give children a taste of what it feels like to be victims of a crime; to teach them the importance of security at school; of not accompanying a stranger alone; to give them an insight into what happens when someone is arrested, charged and faces a trial.
Headteacher Krystyna Piatkowski said: "Each year we choose a programme for the whole school which is relevant to the children's lives and this year it is based on the idea of being safe.
"While the lower school are looking at personal safety, the upper school are concentrating on the responsibility aspect - taking responsibility for themselves, their property and their school."
When she approached the police for help, PC Allan suggested staging a mock crime at school. A few weeks later, PC Steve Blakeley, disguised and claiming to be from Kirklees, the local authority, arrived to collect two faulty keyboards for repair. Having had his identity checked, he was allowed to take them out to his car, with the help of two children. Only when the official repairer arrived half an hour later, did the children realise he was an imposter.
"We made sure he lifted up a mug as he moved the keyboards so we had the chance to do some fingerprinting with the children for elimination," said PC Allan. He also questioned them closely about his description and although they were mainly accurate about his height, build, Nike T-shirt and baseball cap, other details differed wildly. Some were adamant he wore black Adidas trainers, a few said he had a tattoo on his arm and one girl said she had seen him hanging around the playground before he came into school. In fact he wore black leather lace-ups, had no tattoos and had never been to school before.
When the culprit returned to school, this time pretending to inspect new windows, he was quickly spotted and identified by the children.
"This is one way of letting children see at first hand what can happen and what it feels like to have something stolen," said PC Allan. "I always ask them how many felt upset." In this case more than half the hands shot up. One boy had even put his arm round his class teacher and told her not to worry - he did not blame her for letting the man take the keyboards.
"If children show signs of being upset immediately after it has happened, or parents contact school to find out what's going on, we always let them into the secret," said PC Allan.
So how did the children feel about being hoodwinked? Ten-year-old Michael Oldroyd, captivated by the whole exercise, said: "When we saw him again in the yard, everybody was cheering because we'd caught him. I believed it and I felt awful when the keyboards were taken."
Chrissy Ellis, nine, said: "I told my mum when I went home that a man had taken the keyboards. I was upset and cross and she said it was a bit scary. Some people said it was a set-up when he was caught, but I thought it was good. If it ever happened for real, we would know what to do."
For the experience does not end with the "thief" being caught. The following week, working from a script, the children were given the chance to act out a case in the former magistrates court at Dewsbury town hall, playing the parts of magistrates, solicitors, court officials, reporters and members of the public.
"It is very rare for anyone to experience the resolution of a crime, even real victims," said PC Allan. "This way the children at least get a taste of what happens."