"I think my advice to principals now would be 'Don't put your hand on any kid and if they're going to be [volatile] all the time exclude them'," he said.
Exclusion is when children 16 and under are formally removed from the school.
Newman said that, while no charges were laid against either principal, the impact of an investigation was huge.
"It's soul-destroying. You start to question your own judgment, you start to really sincerely think about whether it's worth doing the job because you've got a life, you've got a reputation, you've got a family to take care of."
The child restraint guidelines were designed to support the Education Act which says staff must only restrain a child if the safety of the student or any other person is at serious and imminent risk.
They also require schools to notify, monitor and report on the use of physical restraint.
But Newman said the rules are not clear.
For example the guidelines say you cannot restrain a student who is trying to leave the classroom or school without permission, but you can restrain a student putting themselves in danger, for example running onto a road.
"Does this mean we follow them down the road and then we can leap on them and grab them just before they get run over by a truck? This is the ridiculousness of it.
"The whole problem is that gray area because a child can be throwing a computer around but if they're throwing it in a corner where there are no [other] children you can't restrain them."
Katrina Casey, Ministry of Education Deputy Secretary Sector Enablement and Support, said the ministry had always envisaged reviewing the guidelines, and teachers, principals and other sector leaders were working with the ministry on refining them.
"The changes the ministry made in 2017 were intended to help keep school staff, as well as children, safe by clarifying when restraint can legitimately be used. This is about keeping our children and young people safe at school," she said.
Newman said volatile children had often suffered trauma and abuse that had not been dealt with. This was more common in Northland, he said.
"We're getting more and more volatile children coming through our system and we're not getting the help we need to manage," he said.
Casey said physical restraint or any form of physical contact with students in schools had always been subject to potential legal action and police investigation.
"Physical restraint is a last resort. We offer training for schools on how to create a positive environment in preparation and the challenging behaviour is less likely. We also offer training on ways to de-escalate situations and calm things down."
Newman said he understood police needed to investigate assault accusations but there needed to be common sense so principals and teachers weren't investigated for months.
He said if guidelines could not protect against that, he questioned the point of them.