While the teacher's registration has been cancelled, parents are calling for further investigation into the scale of the misconduct. (File photo)

Parents are calling for the Ministry of Education to investigate a Marlborough college where a teacher took students out of school to have sex, but the ministry has ruled out further action.

The teacher, a woman in her 30s, admitted having sexual relationships with two students aged under 16 at the Blenheim District Court last week, and has interim name suppression until her sentencing date on December 17.

Multiple sources have confirmed to Stuff that many other students were also targeted by the teacher over at least four years, but most were over the legal sex age. Although this is not criminal, it is a breach of the Teaching Council Code and Standards.

While the teacher's registration has been cancelled, parents are calling for further investigation into the scale of the misconduct, how the school handled the allegations, and whether the offending could have been prevented.

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Ministry sector enablement and support deputy secretary Katrina Casey said the ministry would not be investigating the issue any further.

"The police have investigated the case and it has been brought before the court and concluded. We will not be investigating. We continue to offer support to the school as required.

"We would expect schools to act on complaints and concerns from parents about the safety and wellbeing of their children at school.

"We encourage parents with a complaint to speak to the school and the board first."

SCOTT HAMMOND/STUFF On one occasion the teacher had sex with a student in the car park of a primary school.

Parents not happy with the response of the school could contact the Ombudsman or seek legal advice from Student Rights NZ.

Education Minister Chris Hipkins declined to comment on Thursday.

The school's board chairman said the school would not comment further until the court process had concluded.

A man whose son was a student at the school said it was "bordering on ludicrous" there would be no further investigation "just because they're not underage".

"They were there under the school's care. Just because they could give consent doesn't mean it was OK.

"The ministry should care about this, the school has a duty of care to these kids and it failed ... you would think they would pursue it with full vigour, because it could turn out there were heaps of kids involved."

He was worried the case was being treated "more leniently" because the teacher was a woman, he said.

"Maybe they will argue they are trying to protect the victims, but if you don't pursue it to the fullest, we can't even know how many victims there even are."

A police summary of facts said the woman built a close friendship with a boy aged under 16 in 2017. She started taking him out of school at lunchtimes to have sex in the back seat of her car, saying she would fix his attendance if he missed classes.

Sometimes they would have sex after school on Fridays, when she was waiting to pick up her child. She would drive to car parks at reserves around Blenheim, and on one occasion during the Christmas holidays last year they had sex in a primary school car park.

The woman told the boy she loved him, and he believed they were in a relationship. She sent him naked photographs and videos, and told him not to tell anyone about their relationship. It lasted about a year.

SCOTT HAMMOND/STUFF The teacher took students out of school, saying she would fix their attendance records, to have sex in the back seat of her car at parks around Blenheim.

Another student aged under 16 was invited to join them on one occasion, and that student then met the woman for sex on several occasions.

A man whose son refused the woman's advances said education bosses needed to find out what really happened so they could consider ways to prevent it happening again.

"She was doing their attendance records ... she has mentally worked this out, and plotted this ... She sounded like she was really good at what she was doing. That's the sad thing about it, she was actually planning stuff," the father said.

"But how do you keep kids safe from teachers like this? I don't know. They all get checked out by police. And this lady didn't have any red flags or anything."

Perhaps senior staff could monitor the cellphones and internet usage of teachers, he said.

"Maybe the school should look into something like that. If there was something like that it might have never gone this far, it might have saved a few of these boys, it might have been a safety net for these kids."

The school advertised for an acting principal, deputy principal and acting assistant principal this week.