The pupils were shocked when they turned up at Foxhill Primary to find their teacher had been attacked in a violent break-in.
But their parents were more appalled when they learnt the 'crime' had been staged by the school as part of a problem-solving exercise.
Children as young as five were greeted with a distressing scene of 'blood' on the floor, police tape and a female teacher with a plaster on her head.
Crime investigators arrived and pupils were asked to help catch the culprit by looking at evidence.
Many of the 300 pupils at the Sheffield school remained in the dark until receiving an explanatory letter at the end of the week.
Head Nicola Shipman said parents weren't let in on the secret to keep the exercise as realistic as possible but some said their children had been left frightened.
