Witnesses claim the teenager shouted "he's going to stab me" before he was murdered in front of horrified pupils.
Devastated family members this morning said the teenager had “the best heart”. His mum is said to be "devastated" over her son's death.
Speaking to the Evening Standard, an aunt, said: “His mother can’t believe he has gone, she is devastated.
“Quamari had the best heart. He was a wonderful family boy. We just don’t know why this happened.”
Witnesses have described how terrified children ran screaming in panic after the popular youngster was killed as school finished.
Quamari is understood to have shouted ‘he’s going to stab me’ moments before he was attacked.
Police say the victim, who has not yet been formally identified, was found with multiple stab wounds in Willesden at around 3.30pm.
He was rushed to hospital by paramedics, but was pronounced dead shortly after arrival. No arrests have yet been made.
One witness told the Evening Standard: "He was screaming ‘he’s going to stab me’ repeatedly whilst his attacker chased him."
The stabbing took place three miles from where university student Djodjo Nsaka, 19, was fatally stabbed on Friday.
Djodjo, originally from Streatham, south London, was doing a business studies degree at Middlesex University.
He was knifed outside his halls of residence near Wembley Stadium.
There is no suggestion that the two incidents are linked.
Murder Squad detectives are appealing for witnesses to the latest fatal stabbing to come forward.
A Met Police spokesman said: "A murder investigation has been launched following the death of a teenage boy in Willesden.
"Police were called just before 3.30pm on Monday by London Ambulance Serviceto a 15-year-old male stabbed in Doyle Gardens.
"Officers attended and the boy was suffering stab wounds. The boy was taken by paramedics to hospital where he was later pronounced dead. Next of kin are aware.
"The Homicide and Major Crime Command has been informed and are investigating.
"Whilst officers believe they know who the deceased is, they await formal identification."
A record number of 29 teenagers were murdered in London in 2008, topping the previous worst toll of 27 in 2007. There was a steep decline to 15 teenage murders in the capital in 2009.
A total of 19 teenagers were unlawfully killed in London in 2010, while 15 teenagers were murdered in the capital in 2011.
Eight teenagers were murdered in London in 2012, but the figure rose to 12 in 2013, before falling to 11 in 2014. The death toll rose again in 2016 to 19, 15 of whom were fatally stabbed.
Twelve teenagers were murdered in London last year, all of whom were fatally stabbed.