A vial of cocaine used as a teaching aid during lessons about the dangers of drugs has been stolen from a secondary school in Alphen aan den Rijn, newspaper Leidsch Dagblad reports.

The cocaine, owned by the local health board, was kept in a case along with other drug samples but disappeared while at the town’s Ashram College, the paper says.

Both health board and school say they do not understand how someone managed to steal the drug considering the case was locked for most of the day and was kept under supervision.

The lesson was being given to fourth year pre-university students. Police have questioned the pupils but the mystery has not been solved, the paper said.

The school has written to parents informing them of what has happened.