Towards the end of the video, a woman can be seen walking over to a young woman yelling "You touch my sister again, touch her, touch her".
She then points what police allege is a starter pistol gun at the girl's head.
"Get the f*** out of here right now," the woman yells while still holding the gun.
On Monday, Manukau ward councillor and former police officer Alf Filipaina said while the behaviour looked like something out of a movie, he believed it was a "one-off" incident.
Disciplinary action is being taken against students involved in the fight, Ōtāhuhu College told the Herald, and security measures have ramped up at the school.
Police say a starter pistol has since been handed in to police.
"The incident occurred after school hours, was not on school property and the person arrested is not a student of the school," Detective Senior Sergeant Tom Gollan of Counties Manukau CIB said.
The woman is due to reappear on Friday.