Teenage Afghan refugee Hazrat Wali was stabbed to death by a youth, 17, who had hours earlier been convicted of another knife offence, a court ruled on Tuesday.
Mr Wali, 18, died in hospital after suffering one deep stab wound in the attack.
Players in a rugby match between Richmond School and Hampton School watched the injured teenager as he collapsed, the court heard.
The pupil, 17, whose name was not made public, was convicted of manslaughter and remanded into youth detention. He will be sentenced on March 10.
He killed Mr Wali, an aspiring cricketer, on the afternoon of October 12, 2021, in a park in Twickenham, south-east London.
That morning, the pupil pleaded guilty at Wimbledon Magistrates' Court to carrying a knife into a shopping centre, and was given a given a youth rehabilitation order.
A jury at the Old Bailey on Tuesday deliberated for more than eight hours to find him not guilty of murder but guilty of manslaughter, by a majority of 10 to two.
Mr Wali was sitting in the park with Mariam Ahmadazai, a friend, when they were approached by the accused and five other teenagers.
Prosecutor Jacob Hallam said the pupil, then aged 16, began swearing at Mr Wali, who got to his feet and approached him.
He pushed Mr Wali with his chest, and challenged him to “come at me then”. Mr Wali rang a male friend for help, saying he was going to be in a fight.
The accused then produced a 20cm black knife with zigzag-shaped indentations on the blade, the court heard. Mr Wali told Ms Ahmadazai to step away as the pushing continued.
As the confrontation escalated, the accused stabbed Mr Wali in the right side. Fatally injured, Mr Wali grabbed the defendant's jacket and asked: “Why did you stab me?”
He picked up a fallen branch but collapsed soon afterwards.
The court heard that a teacher from a local school gave first aid and emergency services attended but they were unable to save Mr Wali's life and he died about an hour later.
Mr Hallam told the court the pupil admitted in a phone call from custody, which was recorded, that he acted in anger.
The youth agreed he was hot-headed and said it was “just anger”, and stabbing was “a way to release your anger”.
Giving evidence in court, the youth claimed he got the knife out because they were “pushing each other” and he wanted to be left alone.