A third girl filmed the attack and then posted the video on social media. She and two others have been stood down for their part in the assault on Tuesday, and the assailants both immediately suspended.
Four of the Grey High group are in year 10 and one in year 11, all aged between 14 and 15.
The classroom assault continued despite the protests of teaching staff present.
John Paul II High School principal David Sullivan said the victim suffered a facial injury. She was "shaken but OK" and was not at school yesterday.
Grey High School principal Andy England yesterday fronted up to the Catholic school this morning to "unreservedly" apologise to the victim and her whanau, staff and wider school community.
It was also addressed at a special assembly at Grey High today.
"I am appalled. I am deeply disturbed by the behaviour of this small group of students," Mr England said.
"I am upset, disturbed and embarrassed for the good of our school. I never expected anything like this to happen — and nothing like this has ever happened in my time at the school."
He is in his final weeks at Grey High, having resigned to take up the principal’s role at Darfield High School in Canterbury.
Mr England said he understood the assault had been provoked by posts allegedly made on social media.
"Schools are always prepared for the worst. However, nothing can prepare anyone for something like this. We prepare for schools going into lockdown if a nutter turned up, but we don’t consider that students from another school will just walk into a classroom and assault another student. No-one would ever expect that.
"Classrooms are meant to be safe places, like a hospital in a war, like a marae, like a church. A classroom is a space where young people have the protection of their school and the adults in that space and the whole of society’s normal rules that normal people do not ever break."
Yesterday’s assault broke the "sacrosanct" nature of the classroom, Mr England said.
John Paul II High School principal David Sullivan said he was "disappointed and shocked".
He was working with Mr England and the police.
Both principals said that they did not believe the assault fairly reflected the positive relationship between the two schools.
Mr England said: "I do not believe this incident fairly reflects on the wider community of our school, who are as appalled as I am. We will obviously be taking direct action with this group and considering what else we could have done to prevent this incident in order to ensure that such things do not happen in future."
Mr Sullivan said: "John Paul II is a safe and caring place ... What took place on Tuesday should never happen in our schools. Making sure our students, families and school staff are cared for and looked after is our vital aim, especially at this time."
- Viv Logie
