Student Stephen Dudley died of cardiac arrest and suffered from previously undiagnosed heart conditions, the Coroner has reported.
The 15-year-old died on June 6, 2013, following a brawl on the grounds of his school after rugby practice.
Despite attempts to revive him, under the direction of an emergency operator, he was pronounced dead on arrival at Auckland City Hospital.
Coroner Gordon Matenga released his findings into Stephen's death on Thursday.
He says an autopsy showed the teen had scarring and a number of issues around his heart and it was likely the stress of the assault led to a fatal arrhythmia.
Mr Matenga cleared the school of any blame in Stephen's death, saying it had a zero-tolerance policy towards violence.
This is despite accusations from Stephen's family that there is a "culture of violence" at the school, Mr Matenga said.
The school's name and the name of the students who fought with Stephen have been suppressed.
Anti-bullying, anti-violence and personal development programmes are already in place, Mr Matenga said.
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.