Cassius Turvey, a 15-year-old Noongar Yamatji boy, died in hospital 10 days after he was attacked in Perth's eastern suburbs on October 13, 2022.
Jack Steven James Brearley, 23, Brodie Lee Palmer, 29, Mitchell Colin Forth, 26, and Aleesha Louise Gilmore, 23, have denied murdering him.
On the first day of an estimated 11-week trial attended by Cassius's mother Mechelle Turvey, prosecutor Ben Stanwix told the jury the group chased Cassius down.
"He was caught, knocked to the ground and deliberately struck to the head with a metal pole," he told the West Australian Supreme Court on Monday.
"The impact of the pole split his ear in half and caused bleeding on and in his brain."
Mr Stanwix told the jury Cassius' death was "the end point of a complex series of events that had absolutely nothing to do with him".
"He was set upon by people he did not know over petty grievances that did not involve him," he said.
He said the events started on October 9 when Forth, Brearley, Gilmore and another man, Ethan Robert MacKenzie, 20, allegedly "snatched two kids off the street" and unlawfully detained them, punching, kicking and stabbing one of them.
The incident was triggered by a "love triangle" involving Gilmore's 14-year-old brother and another teen of similar age and social media exchanges about the boys fighting.
MacKenzie allegedly pulled out a 35cm knife and stabbed a boy in the chest, causing a shallow wound.
The next incident happened three days later on October 12 when a group of school-aged kids went to Gilmore's home and "almost certainly in retribution" smashed the windows of Brearley's car, Mr Stanwix told the jury during his opening submissions.
"It was tit-for-tat escalation," he said, also describing how Brearley and Forth had later used a car as a weapon and chased down two boys, hitting one of them.
The following day, after Gilmore's brothers warned that a group of teens could be coming to their family home, where Brearley also lived, looking for a fight.
"Intent on violence" the allegedly accused armed themselves with metal poles pulled from shopping trolleys and knuckle dusters before driving off to search for youths.
"Somebody smashed my car and they're about to die," Brearley allegedly said.
At about the same time, Cassius was walking with a group of 19 fellow students to watch a fight being talked about on social media.
The accused intercepted the group and during a series of clashes, Brearley was allegedly slashed in the leg with a knife and another boy was struck in the face with a metal pole.
Cassius and some of the other "terrified school kids" fled into nearby bushland, with Mr Stanwix saying Brearley was "hunting for kids with a metal pole".
"Cassius didn't make it as far as the fence when the accused Brearley caught up with him," he said.
"He was struck to the head at least twice.
"At the moment Brearley struck him he was filled with fury ... he did so intending to at least cause Cassius serious injury."
Mr Stanwix said Brearley later bragged about his "vigilante violence", saying: "He was just lying in the field and I was striking him with the trolley pole so hard so he learned his lesson".
MacKenzie is also on trial but not for the murder charge.
The trial, which is expected to hear from 89 witnesses, continues on Tuesday.
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