Mandatory reporting of hundreds of cases of teachers physically restraining children will show the scale of the behaviour problems they're dealing with, school principals say.
But they warn that teachers are increasingly worried their snap judgements about when to restrain a child could land them in court.
The Education Ministry received more than 400 notifications of physical restraint in schools in the first three months of mandatory reporting, which began on 1 August.
Deidre Alderson from Willowbank School was among several hundred principals who had reported a case of physical restraint of a child since then.
"We had a child whose behaviours were causing the teacher concern and she was unable to de-escalate the problem," she said.
"Her only real alternative was to hold the child so that she could get the rest of the children out of the classroom so that they were safe."
Ms Alderson said the level of restraint that must be reported to the ministry was too minor and the paperwork demands were not helpful, but the requirement to notify the ministry was a good idea.
"We know that there is an increasing number of children across New Zealand with particular behaviour needs and we need the data to know exactly what's going on, why it's occurring, how often it's occurring. So I think the collection of data is wise."
Another principal, who asked not to be named, said children had been restrained at her school twice in the past 18 months, with one case occurring since the requirement for notification.
She said the incidents involved the two groups of children that teachers most commonly needed to restrain.
"They are special needs in one case, a child who got out of hand, and the other is generally children with significant behavioural issues where again they've gotten out of hand," she said.
"The risk has been there for others, for other staff or other children, but they haven't actually taken that step to attack another child."
The principal said making the decision to intervene within the scope of the law was not always simple, particularly when deciding if the risk of harm was imminent.
She said she was aware of some teachers who found themselves under scrutiny even though they had acted with the best of intentions and thought they were acting within the law.
"They've made on-the-spot decisions under quite some pressure often and then have later learned that that defence won't necessarily hold because the threat was not imminent enough," she said.
"When you put staff in a situation like that, there is real potential for people to stop making common sense decisions and step back for fear of a criminal investigation."
The Children's Commissioner, Judge Andrew Becroft, said teachers should not be afraid to use their power to restrain children who were at risk of hurting themselves or others.
"It's proper that that provision is used as and when appropriate and when needed. It's there for a reason, so I'd hate to think teachers felt that if they were legitimately exercising that power that somehow they would be criticised. We trust teachers to do that."