The mother of a girl bashed in an after-school brawl outside a secondary school in Helensville has been arrested after she took on one of the bullies.

Mellissa Anderson, who chased down the alleged ringleader in the attack, has defended her actions saying they were "a mother's rage'' after seeing her daughter hurt.

The group attack involving four girls, two aged 13 and two aged 14, took place between 3.30pm and 4pm outside Kaipara College on Friday.

Summer Anderson, 13, said she was waiting to catch the bus home with a friend when the group of students, also from the school, targeted her.

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"These girls started chasing me down the road and beating me up. They were punching, hitting and kicking me,'' she said.

A resident who saw the incident taking place, took Summer into his house to keep her safe from the girls, where she called her mother to tell her what happened, she said.

Ms Anderson arrived 10 minutes later and was shocked to find her daughter bleeding and distressed inside the resident's house on Awaroa Rd.

"I saw her with blood coming out of her eye from the cuts. She just saw me and cried and my heart stopped,'' she said.

Motherly instinct then took over, said Ms Anderson, who ran after the attackers and confronted one of them in a nearby alleyway.

"I asked if she was the one who hit Summer and she laughed at me and said `yes'.''

"I slapped her a couple of times and I was still holding her when the cops put me on the ground,'' said Ms Anderson who was arrested for assault.

Police also arrested two 14-year-old girls, said school principal John Grant.

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The attack has left Summer with a black eye, a welt on the side of her face and cuts to her eyelid and the incident has left her too terrified to return to school, her mother said.

She has kept her daughter home since the incident and is considering sending her to a different school next year.

"She's pretty scared, she doesn't want to go back.''

Summer said the group attacked her because they believed she was going to fight the cousin of one of the girls - a rumour she said was untrue.

While Ms Anderson believed she shouldn't have confronted the girl, it was an instinctive reaction after seeing her daughter hurt, she said.

"Admittedly a lot of mothers if they had the chance would have done what I did, and felt guilty later like I did.''

She said she planned to plead guilty to assault when she appeared in Waitakere District Court tomorrow.

Mr Grant said while he was "always disappointed when students end up fighting'' it was difficult to prevent such incidents, which he described as rare.

"These things happen. They happen in the adult world and in the adolescent world,'' he said.

Board of trustees chairman Stanley Phillips agreed.

"Kids are kids. Things get blown out of proportion by kids _ we have to let the police and the principal of the school do the investigation but we are aware as a board of society's concerns of violence and Facebook and text (bullying),'' he said.

Student behaviour was increasingly getting worse throughout the country, but it was difficult to prevent such incidents, especially those that took place outside school grounds, said Mr Phillips.

"What can you do about it? We can only do our best.''

The school are investigating the incident with police and would resolve the issue by the end of the school term, which was likely result in standing down or suspending those involved, said Mr Grant.

An anti-bullying programme was already in place at the school along with ``strong student support'', he said.