Independent investigators say senior staff at New Plymouth Boys' High School failed to deal with bullying that had a profound and lasting effect on some boarding students.
The school board called in consultants last November after complaints in the Taranaki Daily News.
Their report says a culture of bullying was found amongst some students in the school hostel, going back more than a decade, including what's described as a nasty assault.
The attacker was made a prefect the following year and senior staff failed to deal with the violence.
A hostel code of silence protected perpetrators and there was no evidence serious breaches of behaviour were dealt with by the previous principal.
New principal Michael McMenamin says there will be zero tolerance of violence, and regular reports made to the board of trustees.