The boy, who was 15 at the time, on Monday admitted to a Victorian Supreme Court jury that he was guilty of manslaughter over Oscar Hamilton's death in September 2024.
But prosecutors rejected his plea, instead taking the case to trial on a single count of murder.
Crown prosecutor Erin Ramsay told jurors Oscar, 16, was killed at Woodgrove Shopping Centre in Melton, west of Melbourne.
Oscar was seated in the centre's outdoor area with his girlfriend and other friends about midday on September 24 when the accused approached and asked to speak to him.
The 15-year-old wanted to confront Oscar over allegations of sexual assault, Ms Ramsay said.
A day earlier, the accused killer's former girlfriend, who he had dated for only a week, had disclosed claims that Oscar had assaulted her.
CCTV footage played to the jury showed the pair talking for about a minute before Oscar turned around and walked back to his friends.
The accused killer followed, pushing one of his friends out of the way and allegedly threatening to stab them all.
The teen then called his ex-girlfriend and asked her if what she had said was true, Ms Ramsay said.
Oscar denied doing anything wrong before the younger boy grabbed him by the collar, pushed him and then stabbed him three times in the chest.
The 15-year-old ran from the scene while Oscar's friends called triple zero and people performed first aid.
Despite their efforts, Oscar died at the scene.
The accused killer, who cannot be identified for legal reasons, called his ex-girlfriend after the incident, apologising for what he had done and saying he didn't mean to do it, jurors were told.
He was arrested at his friend's home later that evening.
Ms Ramsay told the jury there were recorded phone calls between the accused and his family while the boy was in a youth detention centre.
It's alleged he initially told his mother and sister that he wanted to marry his ex-girlfriend before turning on her when she did not respond to his messages.
He proceeded to call her a "bitch", said he "came in here for nothing" and "even if (Oscar) didn't touch her, I would have done it", the jury were told.
The teen's anger over the sexual assault allegations was the alleged motive but it was not for the jury to decide the validity of the girl's claims, Ms Ramsay said.
There was no mention of the girl going to the police or making any formal claim.
Defence barrister Richard Edney told the court his client admitted causing Oscar's death but he was not guilty of murder.
The key issue in the trial would be the boy's intention at the time he stabbed Oscar to death, Mr Edney said.
The trial is expected to run for two weeks.
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