The siblings pleaded guilty to recklessly causing serious injury, with the older brother also admitting to damaging a car before the assault.
“The behaviour of my client is accepted as being outrageous,” defence barrister Tim Smurthwaite told the Victorian County Court on Wednesday.
Jay Stephens took eight Xanax anti-anxiety pills and drank about $300 worth of alcohol on the day of the attack because he was depressed about not seeing his young child interstate, Mr Smurthwaite said.
He had left Queensland while on parole for violent offending in the state, breaching his conditions.
Despite past violent offending, the lawyer argued the 21-year-old shouldn’t have extra jail time on top of what he spent behind bars awaiting sentence.
Stephens also apologised to Mr O’Neill through his lawyer.
“It’s a quantum leap in offending,” Judge Lacava said of Jay Stephens’ latest crimes.
The brothers had been drinking together and threatened tennis players in Malvern East before the assault, the prosecutor said.
On a train shortly before the bashing, witnesses heard the older brother say: “I don’t care if we end up in jail tonight”, with the younger sibling throwing up.
They were kicked off the train at Heyington station, where Mr O’Neill followed them onto the school grounds because he was concerned about their behaviour.
Mr O’Neill was left with broken ribs, bleeding on the brain and in the chest and was off work for several months after the attack.
While the physical injuries would heal, the police officer was left “hypervigilant” and often felt uneasy and on-edge around people.
Isaiah Stephens is being assessed for a community-based sentence rather than jail time.
The teenager was also in a “drunken state” and joined in after he saw his brother involved in a fight.
“He has terrible embarrassment about causing injury to anyone, in particular an old man,” lawyer Simon Moglia said.
The lawyer labelled the attack “inexcusable, unprovoked and troubling”, but said the teen had no prior criminal history and had a deprived background.
“It leaps off the page that his troubled life has left him without the necessary skills and a psychological injury,” Judge Lacava said of a report that left him “very moved”.
But the judge said their sentences must reflect the denunciation of “such thuggery”.
The brothers hugged in the dock before Jay Stephens was taken back into custody.
They will be sentenced later this month.