It must have seemed like a good idea: stage a mock robbery and then ask the pupils to interview the witnesses and write descriptions of the suspects.

Asking local law enforcement officers to play the part of the robbers would give it extra credibility, and they would be on hand to pass on tips afterwards.

But the teacher who organised the “crime” for their forensic science pupils didn’t anticipate a keen-eyed member of the school kitchen staff who was eager to do their civic duty.

The cook alerted the authorities after spotting the police officers pulling on stocking masks outside Montrose High School in Michigan, USA.

About 180 pupils were evacuated from the cafeteria and the school went into lockdown before the robbers were discovered to be Montrose police chief Darrell Ellis and one of his officers.

Once the all-clear had been sounded, the mock robbery, which had been a feature of science classes at the school for 12 years, went ahead as planned.