AUCKLAND, May 19 - In a report released on Thursday, a coroner found 15-year-old Stephen Dudley died of cardiac arrest following a brawl on the grounds of his school after rugby practice in June 2013.

The two brothers who attacked Stephen pleaded guilty to assault, were discharged without conviction and given permanent name suppression after the incident.

But coroner Gordon Matenga in his report accepted the assault was a key cause of the cardiac arrest, despite finding Stephen also suffered from a previously undiagnosed heart condition.

A spokeswoman for the Stephen's family, Ruth Money, told Radio New Zealand the family would now be writing to the solicitor-general to ask for manslaughter charges to be laid.

"We believe the appropriate charge is manslaughter, certainly against the eldest brother, who was very large when you compare the size of him and Stephen," she said.

"It was a very violent, very physical assault... that's been backed up by the coroner's findings."

Matenga cleared the school - which cannot be named - of any blame in Stephen's death, saying it had a zero-tolerance policy towards violence, despite evidence from Stephen's family that there is a "culture of violence".

In his recommendations, the coroner said the school should develop a programme for year 9 students to learn CPR and use an automated external defibrillator as part of first aid training.

He also said the Ministry of Education should set up guidelines for schools around purchasing defibrillators, which would include appropriate training for staff and students.