A 15-year-old boy has been found guilty of murdering Dogan Ismail on the Aylesbury Estate in south London on 30 December.
Ismail, a sixth form student, had gone to the estate with his younger brother to try to retrieve his phone that had been stolen.
After confronting a group of youths, he was then stabbed through the heart and died at the scene - the boy, who cannot be named for legal reasons, said he was going to get the phone for Ismail but returned with a knife instead, the Old Bailey heard.
Prosecuting, Brendan Finucane QC told how the defendant walked straight up to Ismail, stabbed in the chest and walked away.
He went missing after the attack for two weeks but eventually gave himself up and was charged with Ismail's death.
He had denied murder but admitted theft and manslaughter - Ismail's family shouted "yes" as the guilty verdict was read out.
The teenager, who showed no emotion, will now be sentenced for Ismail's murder on 2 July.
Following his murder, Ismail's family said: "Words cannot how much we will miss him. We want justice for Dogan and his killer caught."