Prosecutor Jonathan Turner QC said: "He (the defendant) had been telephoned by his girlfriend and told to come to the estate to teach Junior Nkwelle a lesson.
"There had been an argument between her friends and Junior's friends whilst the football was going on.
"She thought that Junior had insulted her or been less than respectful to her. She was very angry and determined that he should be punished - indeed stabbed - to put things right. She called up her boyfriend and was heard to tell Junior that she had arranged for somebody to come and stab him."
Mr Turner added: "He (the defendant) was encouraged and requested by her to come and do this. Indeed, to use an old-fashioned phrase, was 'set-up' by her to do this."
Mr Turner said the children had been arguing earlier in the evening. It might have been because the boys were thought to be playing too close to flats or that the girls were walking over the pitch.
"Whatever it was, it seems to have been stupid and trivial and completely unnecessary," he added.
The girl did not give evidence but her boyfriend told the court he had instinctively stabbed Junior while they were scuffling.
The girl was found guilty of the manslaughter of Junior Nkwelle, 15, along with her 17-year-old boyfriend.
The pair, who cannot be identified for legal reasons, were found not guilty of murder.
The court heard the girl called her then 16-year-old boyfriend to a housing estate in Brixton, south London, where he killed the keen footballer.
Junior and his friends had been playing football on a grassy area but the girl and her friends appeared to have taken offence.
The youngster was stabbed in the chest after the older boy arrived on a bus and started fighting with him.
He died on the makeshift pitch on the Loughborough estate in Brixton in September last year, after the knife cut his heart and a lung.
Jonathan Turner, QC, prosecuting, said it was likely Junior and his killer did not know each other and there was no quarrel between them.
The pair were remanded in custody at the Old Bailey to be sentenced on April 5.
A teenager will appear in court today accused of the murder of a 14-year-old boy in Lee, south London.Read the full story ›
A 19-year-old man remains in a stable condition in hospital, after a stabbing in south-east London.
It happened in New Cross Road, during the early hours of Sunday.
The victim is thought to have been leaving a nearby nightclub when he was attacked.
No one has been arrested.
Police are still questioning three people about the murder of a 14-year-old boy in south-east London.
Kevin Ssali was stabbed as he got off a bus in Lee on Saturday night.
Kevin who had been missing from home since July, was taken to hospital, but later died. He is the sixth teenager killed in London this year.
A 19 year old man is still in a critical condition in hospital, after he was stabbed in New Cross Road in the early hours of Sunday morning.
He's thought to have been leaving a nearby nightclub.
A post mortem is set to take place after14-year-old Kevin Ssali was fatally stabbed in Lee in south london. Police say he was attacked as he got off a bus. He had been missing from his home in South Norwood since July.
A murder investigation is underway in southeast London, after a 14-year-old boy was stabbed to death as he stepped off a bus. Police say Kevin Ssali was attacked on Saturday night as he got off a bus in Lee. Nick Thatcher reports.
Metropolitan Police have issued an image of 14-year-old Kevin Ssali from Whitworth Road in SE25, who was stabbed to death in Lee.
Met Police have confirmed that the 14-year-old fatally stabbed in Lee in south-east London was Kevin Ssali who had been missing from home in South Norwood since July.