A girl suffered a broken arm during an assault at at St Peter's College in Gore.

A 12-year-old boy who assaulted a girl at his school, breaking her arm, was allowed to remain in the victim's class despite protests from her parents.

The girl's mother, Sally Robertson, who was a teacher at the school – St Peter's College in Gore – has resigned as she and her husband are appalled at the way the incident was handled.

The couple have also removed their daughter, now aged 13, from the school.

Senior Constable Stephen Heyrick said police received a complaint after a boy assaulted a girl in the grounds of St Peter's College on May 15 about 1pm. The incident was filmed on a cellphone by another pupil.

"It was a serious assault that resulted in an injury," Heyrick said.

"He [the offender] has clasped his arms to his chest and charged [from about 10 metres] at a group of girls [who were looking the other way]. The boy who filmed [the incident] certainly knew it was going to happen."

The motive for the assault was unclear, Heyrick said. It appeared the girl was an innocent victim.

Police had spoken to the boy and he was referred to youth aid for further action. Had he been old enough, he would have been charged with assault, Heyrick said.

The girl's left wrist was broken and her plaster cast was only recently removed.

The Ministry of Education was unaware of the incident until it fielded inquiries from Stuff on Thursday.

"Schools are responsible for managing day-to-day incidents of this kind but we can become involved if parents aren't happy with how a complaint is dealt with by a school board of trustees," the ministry's head of sector enablement and support, Katrina Casey, said.

"We've been in touch with the school, which has informed us that it has carried out an investigation into the incident."

Casey said the incident was serious and the "sanctions for the boy involved were also significant".

"It's really unfortunate that it wasn't able to be resolved in a way that the girl returned to the school."

Inquiries reveal that the boy was stood down for several days as part of his punishment. During that time, he did community work.

The victim's parents asked that the boy be permanently removed from their daughter's class because she felt threatened. However, the request was refused.

Casey said in addition to being off school for nearly five days, the boy received six more sanctions. He also wrote a letter of apology to the principal and wanted to offer the same apology to the girl.

"The school made a number of attempts to resolve the issue with the parents of the girl that were ultimately not successful. They made contact with the parents on four occasions, asking to discuss the incident. A restorative justice session was discussed with parents but did not occur."

St Peter's College issued a statement saying it was a strong Catholic school and "the value of compassion and forgiveness is very important to us".

". . . It is not in the interests of the young people involved for the school to make any further statement in relation to it."

The victim's father, Blair Robertson, said he believed the boy's punishment was "weak and pathetic" considering the serious nature of the assault.

The boy should have been formally suspended and made to front a disciplinary hearing with the school's board of trustees, he said.

The couple were particularly disappointed that the school did not remove the boy from their daughter's classroom despite their requests, Robertson said.

"You can't protect the offender and have the victim become more of a victim. It's just ridiculous.

"She didn't want to be in that environment. She felt threatened enough that she wanted to leave [the school]."

Robertson said the incident showed a lack of discipline at the school.

"I think there is zero discipline policy and it needs to be looked at.

"There's nothing we can do about [my daughter] leaving the school and Sally resigning – the horse has bolted on that one – but in future I would like to see them put in a real direct, hard nosed approach to discipline."

The boy's mother declined to comment.

According to the St Peter's College website, the school – established in 1969 – was the first co-educational Catholic school in Australia and New Zealand. It caters to pupils from Year 7 to 13.