A 15-year-old boy who stabbed a student at Ashwood College thought his victim had "dobbed in" some mates for committing a burglary.
The boy, who appeared at a children's court on Monday and cannot be named, had been expelled from the school last year.
He stabbed his 15-year-old victim on the inside of the right forearm with a knife similar to a box cutter last Friday, causing serious injuries that required surgery.
The accused pleaded guilty to charges of intentionally causing serious injury and possessing a controlled weapon. Two other charges were withdrawn.
A police detective said the boy was on parole for a similar stabbing assault on a victim he believed was worthy of "retribution".
He had also previously been found guilty of trafficking and theft, and had been charged with intentionally causing serious injury, recklessly causing serious injury and assault by kicking via summons in relation to an incident in January last year.
The detective said the victim was recovering at home, but was "petrified" about what would happen if he left his house, and had received threatening messages on Facebook from friends of the accused.
The victim may have permanent injuries, but that would not be known for weeks or possibly months. His mother was in court, as was the mother of the accused.
The detective said bail was opposed because of the risk to the victim, and because Youth Justice, which had been monitoring the offender while he was on parole, had also raised concerns about his release.
A magistrate told the defence lawyer that they must show why the accused should be bailed, given the stance of Youth Justice.
"They know him better than anyone else, and if they're not supporting bail today I'd be very reluctant to release him," the magistrate said.
The matter was then adjourned until 2pm, at which time the accused's lawyer decided against applying for bail.
The accused will face court again on February 25. A pre-sentence report will be prepared by Youth Justice.