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".