A 12-year-old boy has been expelled from school after holding a knife to a pupil's throat.
The boy threatened a 12-year-old girl with a pen knife as she waited outside a classroom.
The incident took place on Tuesday at Whitgift School, in Grimsby.
A spokesman for Humberside Police confirmed an investigation was underway and added: "We are treating this matter very seriously."
Head teacher Dr Sheila Kaye told BBC News Online the boy had been permanently excluded from the 1,100 pupil school within a couple of hours of the incident.
Dr Kaye admitted the attack in her school had come as a surprise and said: "It makes us wonder why a student without a record of misbehaviour would do it except as a copycat.
"We, of course, take this very seriously indeed.
"The intent of the boy doesn't matter. You cannot give people another chance in a situation like this."
The victim is due to return to school on Friday.
The incident occurred a week after Lincolnshire schoolboy Luke Walmsley died from a stab wound to the heart.