The incident happened at a school in Vallentuna on Monday afternoon. The pupil had allegedly produced the knife following an earlier disagreement with the headteacher.
“They had been arguing earlier in the day, then later he came back with a knife,” said Anders Olsson of Roslag police.
The police were alerted to the incident at 2pm, after the headteacher was forced to lock himself in the staff room when the boy turned up with a knife. The boy was arrested shortly afterwards and is suspected of making threats and breaking knife laws.