Nathan Brown's victim, Carl Rickard - known as CJ - died pleading for help and asking "What did I do?"
He was left crouched on all fours with blood pouring from his head at the school in south- east London after being hit three times with the 17in bladed weapon. He died two weeks later in hospital from extensive brain injuries.
Brown, then 15, carried out the attack because of an alleged insult aimed at his gang, the Golden Snakes, which modelled itself on the Chinese triads.
It also emerged that pupils at Kidbrooke School, in Eltham, where the boy was killed in January are in fear of retribution from members of the killer's gang for testifying at the Old Bailey trial.
They were allowed to give their evidence behind screens during the case and keep their names secret after Judge Clive Tayler decided "there are real grounds for fear of the consequences if they are required to give their evidence in circumstances where their identity is revealed".
The judge had been told by Orlando Pownall, counsel for the prosecution, that "these are terrified children. Out of 22 witnesses from the school, only one, an especially courageous girl, is prepared to reveal their identity - and two have left the country."
Patricia Jaffe, the head teacher at the school, said in more than 20 years in teaching, she had not seen the "kind of fear that I had to sit through when they gave their statements. Some were in shock, some were in tears - they really found it extremely hard".
The case reveals the worrying rise in the number of attacks connected to teenage gangs.
Brown, now 16, wept and wrung his hands in the dock after relatives of his victim cheered in court as the guilty verdict was announced.
Judge Tayler told Brown: "There is only one sentence - you will be detained during Her Majesty's pleasure."
Brown had admitted Carl's manslaughter but denied murder. He claimed that he had feigned the attack and had never meant him serious injury.
The dead boy's mother, Lyn Rickard, said afterwards: "I am just dead inside.
"I laugh and joke, but it is a dead laugh. I hate getting up every morning. CJ is on my brain 24 hours a day, seven days a week."
Brown was described as a loner who was fixated with martial arts and computer fighting games. The court heard that he had joined that gang to escape the bullying and teasing he endured because he had no father at home.
He recruited his own followers and drew up rules in his diary for joining the Golden Snakes.Reuse content