A 15-year old boy on trial for stabbing Tashaun Aird to death was allegedly caught at school with a seven-inch Rambo knife in his trousers just months before the fatal attack, the Old Bailey has heard.
Giving evidence from behind a screen, his teacher described the "traumatic" moment he had a "tussle" with the pupil and "saw the weapon's jagged edge", before letting him leave the school "for everyone's safety", because no other staff were on hand to help him.
The boy is one of two 15-year-olds - who both cannot be named for legal reasons - who are on trial with Romaine Williams-Reid, 18, of Romford, for the murder of Tashaûn Aird.
The 15-year-old was stabbed to death in Somerford Grove, Stoke Newington, on May 1.
A fourth defendant, Cayden Stuart, died in custody in June aged 16 while awaiting trial.
Jurors heard how the boy apparently "darted off" to the toilets 20 feet away when the teacher told him he needed to be searched with a metal detector wand when he turned up for school on February 27.
"Without being wanded I assumed something was wrong, because students in the past have concealed things," he said.
He found the boy bent down in a toilet cubicle and decided he was "trying to hide something behind the door".
When the boy refused to show the teacher what he was concealing, he decide to "hug him" to get a better look.
"I saw what I can only describe as a large Rambo style knife in a sheath," he told jurors.
"I could see the handle four inches long. I could see the bottom shaft bit of the blade that was silver and shiny and where the long shaft of the knife was in the leather sheath six to seven inches long tucked into his rear waist band."
There was a "brief tussle" and he shouted out to his colleagues for help, and told the boy to hand over the knife.
The boy allegedly refused, saying: "No I can't give it to you. It's not mine. Just let me leave".
You may also want to watch:
When no one came to his aid, the teacher decided to let the boy leave the school because he was "bigger and larger than me".
"The first chance he got he made his way to the security doors and left," he said.
"The importance was he was roaming the streets with a six inch Rambo blade. I got straight on the phone to call the police."
The boy was excluded from school for five days.
In cross examination, the barrister for the boy who denies having had a knife, pointed out that the teacher had made a statement to police an hour after the incident in which he had said the boy "didn't want to hand over his phone" - which had not been mentioned by the teacher in his description of the incident in court.
She suggested he might have mistaken the phone for a knife.
But the teacher told her: "That's wrong.
"I laid eyes on it. I saw the handle. I saw the jagged edge."
Despite working in education for 15 years and coming across weapons regularly which he has often had to confiscate, the incident "left him feeling emotional".
"I've had a lot of experience of students bringing in concealed weapons but not a confrontation," he said.
"I was traumatised by the whole experience, having no support from the staff and having to deal with this all by myself.
"I saw a Rambo knife and was in a vulnerable situation. I did my best to risk assess the situation to make it safe for myself, the pupils and my colleagues."
All three defendants deny murder. wounding another boy with intent and possession of a knife.
The trial continues.