A private care home has been criticised by a judge as he sentenced a mentally ill resident to life for killing an innocent schoolboy.
Serif Aslan had not been taking his anti-psychotic medication and said he had kept a knife in his room for about a week.
As he walked past a school to go to his favourite cafe, Aslan made a remark about a girl and got into a fight with a schoolboy.
Kasey Gordon, 15, went to help the boy and was stabbed through the heart and died on the pavement.
Three other boys were injured in what was described as "a scene of carnage" in West Green Road, Tottenham, north London, in January, last year.
Aslan, 34, was found guilty of murder at the Old Bailey and sentenced to life with a minimum term of 20 years.
He was given concurrent sentences of up to seven years for injuring the three other boys with the knife.
Aslan had lived in the nearby care-in-the-community Ashness House home for six years.
Staff were meant to ensure he took his medication twice a day and to conduct random searches of his room, where knives had been found twice before.
He had been arrested in the past for criminal offences including indecent assault and having an offensive weapon, and suffered from paranoid schizophrenia.
Judge Richard Marks told Aslan: "It seems to me that better supervision may well have avoided the tragic events.
"It is plain that the systems in place at Ashness House to see that you took your medication were plainly wholly inadequate, certainly in the way they were implemented in your case."
The judge said if Aslan had been taking his medication, the amount in his blood would have been four to six times higher, indicating he may not have taken his tablets for a week.
In the previous month, he had become seriously ill with psychotic symptoms and paranoid ideas, leading to suspicions that he had twice not swallowed his pill after rushing to his room.
This should have put those entrusted with his care "on notice" that there was a pressing need to ensure he was taking his medication.
"This clearly did not happen," added the judge.
He said Kasey was a young boy who "happened to be in the wrong place at the wrong time".
Judge Marks said: "The shocking nature of these terrible and tragic events, cannot be over-stated."
Aslan's illness had contributed substantially to his actions and he was to be returned to a psychiatric hospital to serve his sentence.
Impact statements from Kasey's family said they were devastated and would never get over his death.
His mother, Verona, said: "Kasey was the soul of my family. He was a born leader."
She said she was not seeking revenge, adding: "Nor do I have hatred in my heart for the man who took my son's life if he is truly mad."
The trial was told Aslan had been warned against carrying a knife and for his tendency to stare at young women.
He had been walking along the road with a £1.49 kitchen knife hidden in his hand with tissue paper, but he came upon the children.
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."
The 16-year-old boyfriend, who like other surviving pupils cannot be identified, was stabbed in the chest but survived.
He told police in a video interview that Aslan told him: "Your girlfriend is very beautiful."
The youth said: "I told him to move on. He pushed me and said 'I will f*** you up'.
"I knew he had problems. He is always round my school. He is always looking at the girls, always making comments."
A 14-year-old boy was stabbed near his right hip bone and a second 14-year-old boy was treated for superficial cuts to the right side of his face.