Security is to be tightened at a secondary school in Highgate after a boy was injured by an intruder armed with a knife.

Police were called after a middle-aged woman burst into William Ellis School, in Highgate Road, as sixth form students left the building at 10.45am last Friday.

She was pursued by staff as she ran up the stairs and along a corridor where she broke a door, and smashed a glass panel before throwing the knife towards pupils as she left.

Headmaster Sam White praised both staff and students for “remaining calm during the incident”.

The Year 10 pupil who was injured in the attack was back at school on Monday “showing fortitude after what was a very alarming experience for him and others who witnessed the intruder’s behaviour,” he said.

He added: “The woman was waiting in the reception area and as students opened the second security door she dashed in. We escorted her out of the school. Our priority was to get her out of the building. It was about to be break time when lots of kids would be emerging through the corridors.”

The school overhauled its entire security system in 2014, introducing locks on its front gate and two in the new double door reception area.

The following year they introduced a lock down procedure to secure all doors and windows which was not activated while the woman was in the building.

Mr White said: “We take security of the site very seriously. We didn’t activate the lock down “containment” procedure. Should we have done? There wasn’t time to enact it which highlights some of the issues with it.

“It works well during lesson time but during break times not as easily. I’m not convinced it would have helped in this instance.”

He added: “We are talking to other schools about this incident, what the issues are in schools and how to deal with them. We are also seeking guidance from the police and Camden Council.”

Cllr Angela Mason, cabinet member for best start in life at Camden Council, said: “Following the serious incident on Friday we can now reassure schools, following confirmation from the police, that the perpetrator has been detained.

“We are reminding all Camden schools of the importance of site security and to remain vigilant.

“We will review the security arrangements with them in light of this particular incident.”A spokesman for the police said: “Police investigating an incident where a woman entered a school in Camden armed with a knife on September 22 positively identified a potential suspect on September 24.

“She has been detained under the Mental Health Act and is currently receiving treatment at a mental health facility.

“She has not been arrested at this time. Enquiries continue.”