A schoolboy has been stabbed to death in north-west London.
Police have not yet found the weapon
The pupil, 15, had been involved in what police described as an altercation with another boy outside the gates of the London Academy in Edgware.
Terrified staff and pupils witnessed the attack on Thursday afternoon as children were leaving to go home.
Academy principal Phil Hearne said children and staff were devastated at the death of "a young man who had the whole world in front of him".
The killing comes a week before the Home Office launches a national knife amnesty.
Police have launched a murder inquiry and said no arrests had been made and no weapon had been found.
Another boy, thought to be aged about 16, was seen running away from the area towards Salisbury Court.
Teachers saw the boy, described as a promising footballer, collapse and immediately called the police, who arrived within a minute, Mr Hearne said.
A Scotland Yard spokesman said the victim was found at 1535 BST outside Stamford Court on Kings Drive, a short distance from the school, by a sergeant from a local Safer Neighbourhood team.
Ambulance crews took the boy, who has not been named, to the Royal London Hospital in Whitechapel but he was pronounced dead two hours later.
Mr Hearne said: "When I had to announce to people that he was dead, I had very tough senior members of staff in tears.
"Since then it has been absolute shock. This was a lovely young man.
"He was looked up to by youngsters here, terribly well respected, an extremely reliable, hard working, bright young man. A natural born leader.
Friends described the boy as a good and funny person
"We don't know of anybody that would have a grudge against him, and we know our youngsters quite well," said Mr Hearne.
He added that there were no gangs at the school and the attack had left pupils angry.
The London Academy replaced Edgware School in September 2004, and has since received a glowing Ofsted report for delivering "exceptional" results.
On Thursday evening, bunches of flowers had been left outside the school with notes paying tribute to a "good and funny person".
One read: "I will miss you so, so much, I can't believe I have to say goodbye, you will always be in my heart and I will never forget you."
Det Supt Simon Morgan said it was a tragic incident and appealed to anyone who saw what happened to come forward.
"It was busy at that time and people have so far come forward but we would like to make sure that we have spoken to everyone," he said.
The Borough Commander for Barnet, Chief Superintendent Mark Ricketts, said the community was appalled by what had happened and he said it may be necessary to step up security at schools.
"Maybe we need to start making some rather extraordinary decisions, maybe search arches, search wands, the sort of security apparatus you'd see at airports and you've seen recently working very effectively at train stations."
Norman Brennan, director of the Victims of Crime Trust, said the stabbing was "another child murdered, and another family devastated by knife crime".
He added: "We need to bring in mandatory prison sentencing and make carrying a knife as anti-social as carrying a gun."
Last December, Scotland Yard launched Operation Blunt in a bid to reduce knife crime in the capital.