A 15-year-old boy has been jailed for life after he stabbed an A-level student who tried to stop him threatening another boy with a knife.
Robert collapsed near Hackney's town hall
The boy, who cannot be named because of his age, stabbed Robert Levy, 16, in Hackney, east London, last September.
Recommending the boy serve a minimum of nine years, Old Bailey Judge John Diehl said he took Robert's life "viciously and needlessly".
Robert's family expressed outrage at what they saw as a light sentence.
His grandmother said: "Nine years for killing an innocent boy - Lord, that can't be right."
The youth was found guilty of murder following a trial last month.
Sasha Wass QC, prosecuting, said Robert had just left his home when he saw the 15-year-old with a knife having a row with another youth.
"One witness heard Robert say 'You can't do that' and then the defendant stabbed Robert three times," she added.
Robert, who had dreams of becoming an architect, was stabbed in the neck, stomach and shoulder and died before reaching hospital.
Judge Diehl told the defendant: "He (Robert Levy) had done nothing unlawful.
"On the contrary, he was trying to keep the peace and he paid for it with his life.
"He was a young student of considerable promise with a life ahead of him and a determination to succeed.
"Sadly, there appears to have developed in Hackney, along with other areas of big cities, youth gangs and street gangs affiliated to older persons."
The defendant, who gave himself up to police 12 hours after the stabbing, claimed he had acted in self-defence.
He said he had been terrorised by a gang to which the boy he was confronting belonged.