The girl filmed the boys chasing Cashford as she shouted “get him”, the court heard.
After he was arrested, the 16-year-old boy shared footage of the attack with three people.
“This was a carefully pre-planned deliberate and violent attack on someone who was not expecting it and who could not defend himself,” said Natalie Smith, senior crown prosecutor from the Crown Prosecution Service.
“Despite his best attempts to flee, he was relentlessly pursued and attacked, even when witnesses reported he was lying on the ground.”
The three teenagers – who cannot be named for legal reasons – were acquitted of murder on Monday.
They are expected to be sentenced for manslaughter in April.
“These verdicts will change their lives forever,” said the judge Mrs Justice Cheema-Grubb.