The alleged victim died from a knife wound to his thigh after a confrontation at a park behind a school in northwest Sydney on Monday afternoon.
A 15-year-old boy accused of inflicting the fatal wound is in custody charged with murder.
Police arrested a second boy, also 15, over the alleged attack after arriving at his home near the crime scene about 9.45pm on Tuesday.Â
Superintendent Naomi Moore on Wednesday said officers had to break their way into the property where the teenager was with his family, after no one responded to the door knocking.
"The second person who's been arrested played an active role," she said on Wednesday after he was also charged with murder.
When pushed on what she meant by an active role, she declined to elaborate.
The attack was not random, with each boy knowing one another, she said, but did not explain further.
Another three people were also at the scene at the time of the stabbing, police say.
The teenager charged overnight appeared solemn and afraid when he sat silently in a children's court on Wednesday morning.
His parents, seated in court, were distraught with the mother on the verge of tears.
The boy, who cannot be identified for legal reasons, did not apply for bail.
Magistrate Megan Heywood explained the court process to the boy before remanding him in custody until his next court hearing, slated for late January.
According to a police statement read in court on Tuesday, the other 15-year-old allegedly set upon the victim unprovoked and stabbed him in the groin area.
The wounded boy could not be saved despite assistance from witnesses.
"I stabbed another boy in the leg," the boy allegedly later told police in a conversation caught on body-worn camera footage.
Supt Moore said young people needed to understand the consequences of their actions.
"We now have three families in this community that will not have their children at home for Christmas," she said.
The death has rocked the community as friends of the slain teenager continue to leave moving tributes on social media.
"We all miss you so much and you are so loved .... you did not deserve any of this. You had the kindest soul," one of his friends said in a video.
His girlfriend remembered the boy as beautiful, strong and who "deserve(d) the world."
"I wish I could have saved you," she posted online.
Premier Chris Minns said he was heartbroken for the family of the stabbed teen, describing the alleged attack in broad daylight as a "terrible crime".
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