At 12:15, in the school courtyard, Harvey was said to have met the defendant for the final time.
The defendant claimed Harvey had threatened him before placing his hand on his arm.
The jury was told that, thinking he was about to be stabbed, the boy pulled the 13cm-long black blade from his pocket.
At that point, he claimed, his memory failed.
CCTV footage played in court showed him pass the knife to his right hand and stab Harvey in the heart with such force he severed one of his ribs.
For 49 seconds, Harvey stayed on his feet. Fatally injured, he then backed away across the courtyard, in shock, before moving back towards his attacker.
The knife-wielding teen advanced, shouting “come on! What now?” according to witnesses, and Harvey moved away.
One girl, standing just feet away, told the jury she grabbed a younger girl and ran, saying, “he’s got a knife, he’s just stabbed somebody!”.
Harvey leant into the nearby cafeteria, looked at his attacker for a final time, then stepped back into the courtyard, looked down at his chest and collapsed to the floor.
For a moment, he lay alone.
Ms Staniforth heard shouting and ran towards the courtyard: “I told him I was there,” she told the jury, through tears.
Morgan Davis, assistant head, told the court he saw people huddled around Harvey as he headed towards the cafeteria – and the culprit.
“I said, ‘just give me the knife’.”
Mr Davis told the court the defendant looked scared and in shock: “He just kept repeating, ‘you know I can’t control it’.”
He did not resist the teacher as he took the knife from him, the court heard.