Teenage footballer Kiyan Prince cried out for his mother as he lay dying from a stab wound to the heart, a courtroom has heard.
The witness described Kiyan in court as "a good person"
The 15-year-old was knifed after attempting to break up a play-fight outside his north London school.
Kiyan's last words were revealed to an Old Bailey jury by a schoolboy who witnessed the attack on 18 May.
A 16-year-old boy, who cannot be named for legal reasons, denies murdering Kiyan but admits manslaughter.
The witness, whose videotaped account to police was played in court, said Kiyan lifted his jumper and started to panic when he saw blood.
"Kiyan saw the blood on his hand and was screaming: 'Why are you doing this to me?'" he said.
"We were all panicking," said the witness, who cannot be named for legal reasons.
"He fell on the ground and then lots of teachers and everyone just started rushing in.
"I was on my knees going towards Kiyan, and Kiyan was, like, shouting 'Mum' and all that."
Kiyan's parents, Mark Prince and Tracy Cumberbatch, wept as the youth described the stabbing at the gates of the London Academy in Edgware.
He said Kiyan, who played for Queen Park Rangers' youth team, was "a good person".
"He gets along with everyone. He don't have no enemies or nothing. He is an easygoing person," he said.
After intervening in the scuffle between the defendant and another boy, the defendant allegedly jumped on Kiyan and got him in a headlock, the witness said.
He claimed the accused, who is a year older than Kiyan, told him: "You think you are big so you can come and stand up against me."
The witness said he saw the defendant produce a silver-bladed knife with a red handle.
"He stabbed Kiyan and then I think he done it a second time, and then the third time the other boy dropped on top of the defendant and grabbed him, trying to get the knife away," he said.
The trial continues.