Amari Smith, from Penfold Close, killed Louis Ryan Menezes in a sudden confrontation on Drayton Walk, Kingsthorpe, in May last year.
The two boys argued with each other over "some comments made on social media", and witnesses described how they "got in each others' faces".
But the verbal argument turned deadly when Smith pulled out a hidden kitchen knife and stabbed Louis once in the heart before running form the street.
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Louis, now fatally injured, was helped to a nearby staircase by a friend but died soon after. He was 17.
Smith later told the jury he believed Louis was about to stab him first, and evidence proved Louis had his own knife tucked into his waistband when he died.
But at Northampton Crown Court today (September 25), Her Honour Judge Adrienne Lucking called the attack a "genuine but mistaken and unreasonable" belief that he was defending himself.
The judge said: "I am sure that what you did in stabbing Louis Menezes once to the chest went beyond the use of reasonable force.
"You left the scene and did not call for help... You successfully and efficiently disposed of your clothing and knife, which have never been found... You did not show any sign of remorse during the trial."
The sentencing comes after Smith was acquitted of murder in August but unanimously convicted of manslaughter by a jury.
Louis Ryan Menezes, 17, was an aspiring motorbike mechanic and had been in Northampton that weekend to spend a night with his friends.
When his family last saw him, the teenager was "bubbly and happy" and looking forward to his weekend.
His last words to his mother Cheri, who dropped him off in Drayton Walk just hours before the stabbing, were: "I love you, mum."
In an exclusive interview with the Chronicle and Echo, Cheri said about Louis: "He was the funniest boy. His little sisters adored him. Everyone just mourns him with such deep grief."
Louis’ family have spoken of the pain of losing him in a statement handed out after court:
“Louis was a young man with his whole life ahead of him. He was a treasured, caring son, brother, grandson, nephew and cousin whose life was cruelly cut short. He is so sadly missed by all his family and friends.
“No parent or family should have to go through losing a child in such a horrific way. Louis’ death has left a huge gap in our lives and the pain of losing him is so great it will never go away.
“We’d like to take this opportunity to thank the Kingsthorpe community for their immense support and kindness, the emergency services for their help and efforts on the afternoon Louis died and, last but not least, family and friends for their love, support and loyalty. Without this, the last few months would have been unbearable.
“We’d also like to thank the detectives, Crown Prosecution Service and the courts for their tireless work in getting justice for Louis and for us.”
Smith was sentenced to nine years and four months in a young offenders' institute.
The court heard, however, that the starting point of Smith's sentence before mitigating factors would have been one of 14 years - the highest category of prison term available for manslaughter.