He said the perpetrator of the assault was known to the school but the other two males were not and the school had left their identification to the police.
"Both myself and the deputy principal contacted the parents of the perpetrator of the assault, and informed them of our disgust at their son's behaviour. The police were informed, and the school has also trespassed the boy.
"This is a very unusual event at our school and we are deeply disappointed that one of our students should be treated in this way."
A letter outlining the incident was sent to the entire school community on Friday following the completion of their investigation.
In it, Fenton condemned the uploading of the video and the social media engagement with the incident and said the school was committed to the safety of its students.
"I am highly disappointed in this whole sad scenario; I am sad for the victim of the assault and his parents, I am sad for the parents of the perpetrator, who are extremely embarrassed, and I hold in contempt the attacker and his accomplices for their actions."
The incident follows another assault at Napier Boys' High School nearly two weeks earlier where a widely circulated video on social media showed a student being struck and knocked unconscious by another student.
At the time New Zealand Principals' Federation president Cherie Taylor-Patel said there had been a rise in the number of assaults being filmed and posted on social media during Covid.
"These incidents are becoming more common, but we need to push back and ask why this is happening. What's causing the aggression?"
Netsafe's online safety operations manager Sean Lyons said in response to the Napier Boys' High School incident that he hoped schools would alert non-profit online safety organisations like Netsafe about violent videos uploaded to social media.
He said they were potentially harmful for those being filmed and viewers.
"Some people record them to be helpful bystanders, so it can be used as evidence. In those cases it's laudable.
"But in some cases, videos are used to try and shame or threaten or harm."