Police are investigating after a video emerged showing two junior pupils attacking a senior, as others look on.
On Monday, the pair were suspended.
School principal Judith Taylor said the board disciplinary committee met on Wednesday night and decided to exclude the pupils permanently.
"This decision reinforces the Board's desire to create and maintain a safe and welcoming space for our students and teachers," she said.
"As part of this legal process, the college is required to try and arrange for the excluded pupils to be enrolled at another school.
"If this is unsuccessful, the Ministry of Education will then take over the process."
Other pupils watched on and filmed the incident.
In a letter to the school community, Taylor said the school was "working with" pupils who filmed and or encouraged the attack.
She thanked parents and caregivers who had sent wishes of support to the victim of the assault and her family.
"She has returned to school, is happy to be back, is surrounded by good and caring people, and is settling in well."
Upper Hutt College was a safe school, she said.
She asked parents to talk to their children about the sharing of harmful images online, and to encourage young people experiencing harm at school to speak up.
It launched an anonymous online reporting system this week, and is already following up on a small number of initial reports already received.
