It's understood the incidents took place at Fell House boarding hostel.
West Coast parents contacted have declined to talk at this stage.
School principal Gary O'Shea did not respond to phone calls from The Westport News.
O'Shea and board chairman Dennis Christian released a statement on Wednesday, saying 13 boys were involved, three of them in Year 13.
One Year 13 student had been removed from boarding and suspended from school pending a board of trustees hearing.
Another Year 13 student had been removed from boarding and been sanctioned.
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.
A third Year 13 student had been stood down.
The incidents had happened at the college's boarding hostels during the first term, the statement said.
An investigation had begun after senior administrators were alerted at the end of the school holidays.
"These 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 ..." the statement said.
"We are deeply saddened that a small group of boys have failed to assimilate what has been a central cultural message and focus within the college, which is the care and respect for each individual."
The college was also considering the most appropriate way to involve the police.
The statement said the school had extensive programmes based around respect for each other and a Year 13 leadership programme to promote personal growth and support for junior students.
The statement 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.
The Ministry of Education's deputy secretary, sector enablement and support, Katrina Casey, told The Westport News the ministry had been made aware of the bullying allegations by a lawyer representing one of the families.
"As soon as we were made aware of the allegations we contacted the school to find out what they were doing to address the issue and offered our support. To date they have not asked for our assistance," Casey said.
She declined to answer specific questions about the bullying, saying the newspaper should contact the college.
She said students had a right to go to school in a safe environment.
"Evidence tells us that bullying can be most effectively reduced through preventative approaches."
The ministry provided guidelines on how to prevent and respond to bullying.
- The Westport News, with additional reporting by the NZ Herald
