The death of Leroy James was recorded on his cellphone, which he handed to a girl before the fight, The Independent reported. The jury in the Old Bailey viewed 12 seconds of footage that ended with the victim's T-shirt covered with blood from a knife wound to the heart.
The defendant's name cannot be used. He has denied the killing and the charge of using a knife.
Duncan Penny, the prosecuting lawyer, said the two boys, both 14 at the time, had gotten into a dispute, possibly over a girl. He said others in the playground in the Ponders End neighborhood in North London were encouraging them to fight it out.
"To those who were watching what was expected to be a fist fight transpired to be anything but," Penny said.
The footage shows the defendant with a hand in his pocket. Penny said the pocket also held his knife and told the jury he was known to call the weapon his "pet" and "alligator."