A teenage boy chased and threatened another pupil with a knife in front of his classmates.

Police are investigating after a teacher at Dorothy Stringer School in Brighton had to intervene to remove a knife from a Year 10 pupil who was apparently trying to attack a classmate.

The teacher is believed to have seen the knife in the 15-year-old boy’s hand from across the school field as pupils were preparing to leave school on Monday afternoon.

The boy involved is believed to have been excluded.

Members of the school community have accused headteacher Richard Bradford of trying to cover up the incident.

In a letter to staff, Mr Bradford told teachers not to speak to pupils about the incident.

He said: “Clearly this was a very disturbing and frightening incident witnessed by a lot of students. It needs very careful management.”

But one whistleblower told The Argus: “The school seem keen to hush it up.

“The head should be condemning this publicly in no uncertain terms and leading on the issue, not telling the staff that they cannot discuss it with students in the hope that it simply blows over.

“People feel very worried about this. Unless something is done to send a clear message, the worry is that this type of behaviour will just continue until someone is badly injured – or worse.”

The school issued a statement to parents after being contacted by The Argus.

In a statement to The Argus, Mr Bradford said: “Police are currently investigating an incident on Monday at the school where one pupil allegedly threatened another with a knife.”

But in the letter to staff seen by The Argus, Mr Bradford wrote: “[A teacher] was walking down from the main school building to the school’s on-site bus waiting area to help manage our students on to the school buses when he saw [A Year 10 boy] chasing after [another Year 10 boy] across the fields. He heard some other students shouting ‘He has got a knife’. [The teacher] could see him in the distance and clearly saw that he was chasing after [the other boy] with a knife in his hand.

“The two boys came to a halt near a group of other boys and [the teacher] stood between them to try to stop a fight.

“[The teacher] saw that [the boy] still had the knife in his hand. He took hold of his arm and asked him to release the knife, which he did. [The teacher] kept hold of the knife and then took him away from the other boys in order to diffuse the impact of the incident and walked him back up to the school buildings.”

A spokesman for Sussex Police said: “Shortly before 3.30pm on Monday, police were called to Dorothy Stringer School, Loder Road, Brighton, where a 15-year-old boy was alleged to have threatened another 15-year-old boy with a knife. No one was hurt.

“The circumstances are being investigated.”