The father of dead schoolboy Stephen Dudley says his family were the "the only ones sentenced today".

Speaking outside the High Court in Auckland immediately after the boy who admitted assaulting his son was discharged without conviction, Brent Dudley said it was "no wonder we have a culture of youths getting away with murder".

"It was absolutely brutal what they did to our son," he said.

"They've both had name suppression, they both now walk away without a conviction.

"It's all been one-way."

Stephen, 15, died of a heart condition shortly after being assaulted by two boys, then-aged 15 and 17, after rugby practice in July last year.

Justice Helen Winkelmann said Dudley had a pre-existing, undiagnosed heart condition called cardiac sarcoidosis and his subsequent death could not be taken into account in the sentencing.

The incident was accordingly treated as a low-level "school-yard fight".

The older boy, now 18, appeared for sentence today after pleading guilty to assault with intent to injure.

Justice Winkelmann began her sentencing by indicating she was going to discharge the boy without conviction.

Dudley's father Brent, who had earlier delivered an emotional victim impact statement, erupted in disbelief, yelling from the back of the court: "You're f ... ... joking."

"You've got to be joking, this guy beat my son to death," he yelled to the judge.

As he was led from the court he yelled "That's justice for you in New Zealand. The law's an ass."

Justice Winkelmann said the older boy saw a fight involving Dudley and the younger boy and ran over and hit Dudley in the neck with a swinging right hand. He landed a few other blows before being dragged off the unconscious boy.

Dudley died but an initial charge of manslaughter was dropped after his heart condition was discovered in an autopsy.

The boy pleaded guilty to assault with intent to injure after a charge of manslaughter was dropped.

The younger boy was discharged without conviction.

Brent Dudley told the court his son was a young man who had a love of life and always saw the good in people.

He said Stephen was a peacemaker and not a violent person.

The assault was an "act of cowardice and brutality".

SUPPLIED STEPHEN DUDLEY: Died after being punched at rugby training.

"He was smaller than you and you attacked him from behind," he told the accused.

Dudley said the boy continued the assault even after Stephen was unconscious.

"You are a coward and any thoughts of forgiveness are entirely out of the question."

Dudley said he considered the boy to be "the hand of evil".

"I've recently seen you on TV playing rugby - wasn't that nice for you?" he said.

"I've followed your Facebook page and I don't see much remorse in that."

Dudley said he got up early in the morning and visualised Stephen coming out of his bedroom.

Dudley's sister Talita told the court her brother was an important figure in her life.

He was a "kind, strong, inquisitive young man" who was a role model to his younger brothers.

Stephen's mother Mona Dudley said she "no longer felt safe in parts of our community".

"I have never felt so robbed of my heart and soul," she said.

Defence lawyer John Munro said his client sought a discharge without conviction.

He had "misread a situation in a very tragic way". He had no explanation for his offending and it was the over-reaction of an older boy protecting a younger boy.

Munro said the boy had displayed heartfelt and sincere remorse and had taught the lessons he had learned from the incident to other young people at his church.

Having a conviction for violence on his record could prevent him from realising his goal of becoming a teacher, Munro said.

Though the Crown opposed a discharge, Justice Winkelmann said it was "school-yard violence ... and we don't usually haul those boys before the court".

The judge said they were "punches thrown in a school-yard fight" and had Dudley not died it would have been dealt with by the school.

The fact the 17-year-old was protecting the younger boy, however misguided that was, showed that he had not "maliciously" involved himself in the events, she said.

She discharged the boy without conviction.

Outside court, Brent Dudley recommended people support organisations like the Sensible Sentencing Trust.

"Too many young criminals and thugs are getting away with murder," he said.