Several Nelson College students have been stood down after "serious acts of bullying".
Organised fights are believed to have happened "half a dozen" times at the college's Fell House boarding hostel late at night during term 1.
Thirteen boys, including three boys in Year 13, were reported to be involved.
A source at the college told Fairfax that a group of junior students were encouraged to fight by two seniors after lights-out.
It is believed one of the senior students was a member of the school's first XV and has been sanctioned in relation to involvement in the school's rugby programme.
Other sanctioned students were removed from the boarding house and suspended from school.
"The incidents were serious acts of bullying perpetrated by a small number of senior boys and in no way represents the values and core culture of the school or our boarding institution," Nelson College headmaster, Gary O'Shea, and board chairman Dennis Christian said in a letter to parents and staff yesterday.
"We are devastated that a small group of boys have failed to assimilate any of this message, which in turn has had a negative impact on some of our junior boarding students."
The letter assured "significant" correspondence had occurred between the school and the parents of the students involved.
Support and guidance for the junior students involved had been put in place.
"Such matters are deeply unsettling to any school community and we very much hope to be able to move on from this in the most constructive possible way," the school said.