The teenager, who wore a mask, had lain in wait for Quamari outside Capital City Academy in Doyle Gardens.
When he saw “happy and hardworking” GCSE student Quamari leave the school he chased him, armed with a large kitchen knife.
The 15-year-old victim ran for his life, shouting for help and saying he was going to be stabbed.
But the killer, who cannot be named due to his age, caught up with him by the school gates and stabbed Quamari three times.
Quamari was taken to hospital where he told a nurse who had attacked him before he died from his injuries.
Witnesses recognised the killer and he was arrested the next day.
The boy was found guilty of murder following a trial at the Old Bailey.
The trial heard the motive for the attack was unknown.
But since his conviction, the defendant has admitted for the first time he carried out the attack and he knew it was Quamari.
However, prosecutor Sally O’Neill QC said: “It is not accepted that Quamari was anything to do with any sort of gang.
“Information from the school painted a picture of a happy, hardworking, well liked and sociable boy.”
Quamari’s mother Lillian Serunkuma described the killer’s actions as “pure evil” in a statement read in court.
She wrote: “You never gave Quamari a second chance to defend himself.
“You took his life in a cold and malicious way.”
She said her son had a “fun loving spirit” and his life was stolen for “no reason”.
Ms Serunkuma said Quamari loved school and what the youth did was “indefensible”.
Thousands attended a vigil outside the school on learning of the death of Quamari, described as a ‘shining star’.
Thousands more turned out for Quamari’s funeral on March 10, filling the Sacred Heart Church, in Quex Road, in Kilburn, to hear family tributes to the ‘much-loved boy with a heart full of gold.”
The court heard the defendant had a string of convictions, including for punching a girl.
In a statement expressing remorse, he said: “I want to say that I’m sorry for what I did.
“I don’t know why I did it. I was scared and confused.
“I’m telling the truth for Quamari’s mum and dad. I’m sorry.
“I didn’t mean Quamari to get so hurt.
“I’m not a murderer. I’m not a waste man. I didn’t want him to die.
“I want to have a different life but I don’t know how. I’m trying.”Judge John Bevan QC said it was “infinitely depressing” to sentence a young person for such a serious crime.
The fact the defendant had finally admitted his guilty would be “a comfort” to Quamari’s parents.
He said: “It is very unusual to admit a murder after conviction.
“It is a mature decision rather than taking your chances in the Court of Appeal.”
The judge sentenced the boy to be jailed for at least 14 years.
He said: “This is a bad case of its kind because Quamari can have done nothing to merit an attack of this severity.
“His death was a product of a total lack of self control combined with the cowardice of knifing an unarmed victim.”
Quamari’s father Paul Barnes had wept in court as the boy was sentenced.
Outside court, he said he was “happy” with the sentence, although it will not being his son back.
He said he was hoping for longer to “deter the kids”.
On the defendant’s admission of guilty, Mr Barnes said: “Personally, I think he was just grabbing at straws, trying to save his own skin. Last ditch dot com. Trying to save his own bacon.”
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here