Police are investigating a fight at Oamaru's Waitaki Boys' High School which ended with a pupil being head-butted and kicked in the head while lying on the ground.
Video footage of the fight, described by police as "very serious'', has been sent to the Otago Daily Times and the school.
The fight appears to have arisen from a dispute on social media.
In the video, two pupils can be seen shoving each other until one lunges at the other.
He makes loose contact and the other pupil then head-butts him, causing him to fall to the ground. He is kicked hard in the head by the other pupil, who then walks away.
WARNING: VIDEO CONTAINS GRAPHIC VIOLENCE AND COARSE LANGUAGE THAT MAY OFFEND SOME VIEWERS
The school's rector, Clive Rennie, said the fight occurred on school grounds on Thursday and was broken up by staff.
He could not say what disciplinary action would be taken but did say a school investigation into the incident was under way.
"I just have to make sure I work through all the processes and evidence.
"I have a reasonable idea of where I'm going to go.''
Mr Rennie described the fight as a "very serious offence''.
He said it was an isolated incident and had arisen from exchanges on social media.
"[The fight] wasn't organised. It was something that just grew. It began with words.
"I'm just amazed that people say the things they do about each other, not seeming to appreciate that at some time they're going to have to face each other and resolve what they've said. They just seem to say what they like.''
He said one of the pupils sustained injuries and received medical treatment outside of school.
He declined to identify the teenagers involved in the fight but the ODT understands they were senior pupils. .
Senior Sergeant Jason McCoy, of Oamaru, said police were investigating the incident.
Mr Rennie said he was "pretty sure'' a pupil filmed the fight, but did not think the boy with the camera would face disciplinary action.
"It's a very serious situation.''
Mr Rennie believed the person who circulated the footage damaged the school's reputation, but the fact it was made so public would not alter the school's disciplinary process or investigation.
He welcomed the police presence and said it helped drive home to pupils that their actions had consequences.
"Hopefully there are lessons in there for people about what happens when you allow something like this to escalate.
"What they have to learn is there are real consequences in school and outside.''
A report made public in 2014 stated that amongst other issues, Waitaki Boys' High School had problems with pupil-to-pupil bullying, pupil-to-staff bullying and staff-to-staff bullying.
