A knifeman who left a "scene of carnage" after attacking a group of pupils near their Tottenham school has been found guilty of murder.
Kasey Gordon, 15, died after being stabbed in the heart. Three other boys were injured.
Serif Aslan, 34, a Turkish man with mental health issues, claimed self-defence but was found guilty at the Old Bailey.
He was remanded in custody and will be sentenced next week.
Aslan lived in a care in the community hostel near Kasey's school in north London.
He had been warned against carrying a knife and over his tendency to stare at young women.
In January last year Aslan was punched by a schoolboy who had been pushed after Aslan made a remark to his girlfriend.
Aslan, who is diagnosed with paranoid schizophrenia, left the scene and returned after buying a £1.49 kitchen knife which he hid in his hand with tissue paper.
He stabbed the 16-year-old boyfriend and other boys, including Kasey, who tried to help him.
Witnesses said Aslan calmly walked away after the attack in West Green Road leaving Kasey bleeding to death on the pavement.
Richard Horwell QC, prosecuting, said: "This main part of the incident lasted about 30 seconds and, when it was over, the defendant calmly walked away.
"He left behind a scene of considerable carnage. The knife that he had carried - the knife he had been told not to carry - was used by him to devastating effect."
Aslan, of Phillip Lane, Tottenham, was found guilty of murder, two counts of wounding with intent to cause grievous bodily harm to boys aged 14 and 16, and causing actual bodily harm to a 14-year-old boy.
The 16-year-old boyfriend, who like other surviving pupils cannot be identified, had been stabbed in the chest but survived.
He told police in a video interview that Aslan told him: "Your girlfriend is very beautiful.
"I told him to move on," said the youth, who then told Aslan to go away.
He said: "I knew he had problems. He is always round my school. He is always looking at the girls, always making comments.
"He pushed me three times and we started fighting. I hit him in the face and he backed off.
"Then he came after me. He tried to attack me and I tried to defend myself.
"I am still fighting because I can't feel anything. Then three black boys got involved.
"He is flinging his arm everywhere. After that I think the man just walked off.
"I lifted up my jacket and I saw I got stabbed."
He added: "I didn't tell anyone to start fighting. After everything had finished, two black boys dropped on the floor saying they had been stabbed."
The 14-year-old victim was stabbed near his right hip-bone and the second 14-year-old was treated for superficial cuts to the right side of his face.
Witnesses, including some of the schoolboys, had not seen the knife and did not realise what was happening.
"It is more than likely Kasey Gordon did not see the knife. His inability to see it and what he was doing with it probably cost Kasey Gordon his life," said Mr Horwell.
A few months before the attack, Aslan had been caught with a knife in his room.
Police were called and warned him against carrying a knife in a public place.