Five pupils have been referred to the Family Court and a sixth will appear in Lower Hutt Youth Court.
The court cases relate to an incident at the school during lunchtime on December 6 when a pupil, aged 14, had his pants pulled down and was violated with an object.
Police say the victim was left traumatised by the attack.
Detective Sergeant Brendan Mears confirmed the investigation yesterday and said inquiries were continuing into a series of assaults, also involving weapons, in the school grounds at Hutt Valley High.
"This attack was bullying in the extreme," he said.
"A group of students have unfortunately developed a pack mentality in carrying out unprovoked assaults on other students."
The Dominion Post has learned the school was told of the assaults on December 7 but did not notify police.
The victim and his mother complained to police and the school after a medical examination three days later.
Mr Mears would not discuss the nature of the assault but said he did not believe it was sexually motivated.
He said the alleged culprits had embarked on a fortnight of escalating schoolyard violence. The investigation had now been expanded to include serious assaults against other pupils.
It is understood that a variety of weapons had been used, including cellphones, scissors and a shoe.
Hutt Valley High board chairwoman Susan Pilbrow said she had understood the assaults were minor, so they were not referred to the board for disciplinary action.
Acting principal Steve Chapman dealt with the assaults by standing down the pupils.
The matter was not referred to the board, as is required for serious cases of assault requiring disciplinary action, such as suspension or expulsion.
Mr Chapman said yesterday that he had no regrets about how he had handled the assaults: "It wasn't an assault where somebody had blood spilt."
The pupils who were stood down would be welcomed back when school resumed on February 5.
Mrs Pilbrow said she had assumed the assaults were minor because they were not referred to the board.
"I feel confident that serious events and incidents come to the board to be dealt with - and those didn't come to the board."
Bullying was not condoned and Hutt Valley High was serious about its role in providing a safe environment for learning, she said.
An Education Ministry spokesman said officials were alerted to the allegations last year and were "working with the school".
- The Dominion Post