The victim, a boy aged 17, died from stab wounds to his thigh after a confrontation at a park in Sydney's northwest about 4.20pm on Monday.
About three hours later, the younger boy arrived at a police station with a parent and was subsequently charged with murder.
The 15-year-old, who cannot be identified for legal reasons, faced a children's court on Tuesday morning.
He sat quietly in the courtroom, wearing a black hoodie and sporting a mullet haircut with shaved sides.
Police asked the court for an order to take forensic evidence from the teenager, including a DNA swab.
A court order is needed to take DNA evidence from an accused juvenile because they are not considered capable of giving informed consent.
The boy did not apply for bail and was remanded in custody for eight weeks, when the case returns to court.
The stabbing happened in broad daylight, close to several schools and within sight of numerous people who were walking nearby.
Authorities remained at the scene on Tuesday as locals laid flowers in tribute to the slain teen.
Police said those involved in Monday's confrontation weren't from the nearby high school and officers believe it wasn't a random attack.
Several female witnesses who were in the park at the time rushed over to help the victim.
Paramedics were called but the teen died soon after.
"It would have been a horrifically confronting situation for those people and I can only praise them for their efforts," Detective Superintendent Naomi Moore told reporters on Monday evening.
"It was a potentially dangerous situation they were walking into, but they thought nothing of themselves to go in and render aid."
NSW Premier Chris Minns sent his condolences to the teenage boy's family and friends.
"I am deeply saddened by the death of a teenage boy at Rouse Hill and extend my sincere condolences to his family and friends," he said in a statement.
"It is an unimaginable loss, and the whole community will feel the impact."
Local federal MP Michelle Rowland said she was deeply saddened by the incident.
"There is no place in our community for these acts of senseless violence, and I know this news will be particularly distressing for local families," she said.
The accused murderer is scheduled to return to court on January 23.
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