A "violent altercation" took place at Whangārei Boys' High School on Wednesday after a non-student went onto school grounds wearing the school's uniform.
In an email to parents, acting principal JT Thomas described the incident as "disturbing for us all".
"We had a young person come onto the grounds, in WBHS school uniform, the result of which led to a violent altercation."
"Thankfully, all parties involved did not sustain any serious injuries. To be cautious we have had medical consultations take place."
Thomas said the police had been notified and "all parties identified will be followed up by the police and the school as appropriate".
"We wanted to reassure you we are doing all we can to resolve the situation as soon as possible as we absolutely do not condone violence of any kind."
Staff were aware footage may be shared on social media, Thomas said.
Thomas confirmed to the Advocate that an incident took place, but provided few further details.
"There was an incident in school yesterday. We are following up through our school process, alongside this the police have been informed and the matter will also be followed up by the police."
A police spokesperson said police were investigating, after they were "made aware" of an incident on Kent Rd, Regent at about 3.50pm on Wednesday.
One person was assaulted, police confirmed.
"Enquiries are ongoing and police are speaking to those involved," the spokesperson said.