Hazrat Wali was attacked in Craneford Way Playing Fields in October 2021.
He had been sitting with a female friend when he was attacked.
The 17-year-old, who stabbed the victim in the right side of his body, was cleared of murder.
He claimed he had acted out of self-defence, but the jury at the Old Bailey heard he had a history of violence and carrying knives.
He cannot be named for legal reasons and is due to be sentenced on March the 10th.