But he confirmed there had been "more than one incident of undesirable behaviour with cellphones".

"We are certainly aware of incidents that come under the Harmful Digital Communications Act. Police and Netsafe are involved," Hill said.

He added: "We have sent boys to the board of trustees for incidents involving cellphones."

The Herald understands that one boy took an upskirt photo from his personal cellphone and circulated it among his peers.

Another boy filmed a video of a female staff member teaching in the classroom and uploaded it to an internet porn website, it's understood.

Hill denies claims the school has failed to act promptly.

"The first priority has been to protect the wellbeing and interests of victims. I'd refute a delay," he said.

The school has responded by clamping down on cellphone use in the classroom.

In a missive sent to parents and caregivers, seen by the Herald, Hill said the school would no longer allow cellphones to be used in class "unless the boys have had a clear and specific instruction from a teacher".

"The teacher is in control of the classroom," Hill wrote, after telling pupils of the cellphone crackdown during a special assembly.

"This decision has been made after some incidents involving the use of cellphones in class that are contrary to the school values and that do come under the Harmful Digital Communications Act. I am also aware of concerns about a state of continual distraction that some students seem to be under.

"I have been heartened that students have reported the recent incidents to me. This confirms that most of our students have a clear moral code and will take a stand when this is crossed."

Police have been approached for comment.

Netsafe, New Zealand's independent, non-profit online safety organisation, said its service under the Harmful Digital Communications Act was confidential and would not confirm whether it had received reports about specific incidents.

Hill has asked parents to talk to their sons about their use of social media and to monitor it, along with their cellphone use in general.

"The underlying purpose of school is that caring adults [teachers] get alongside students and act as high quality role models. This purpose is enabled by a culture of respect," Hill wrote.

"I do hope that students are able to respect the role of the teacher and the school's position on cellphones."

Students will continue to use their school approved devices and the school network in class for the purposes of "enhancing their thinking and engaging with a 21st Century curriculum".

Hill said it was important not to blame cellphones.

"We talk to the boys about being good men, not real men. And let's not blame the devices. Boys, at times, have got things wrong," he said.

"We've got strong values. It's all very well to have the values, but at times you do have to take some action."