Fears legalising cannabis will lead to "drugged out students" in classrooms has a prominent Rotorua principal calling for a rethink on the impending referendum.
A public referendum on legalising cannabis for personal use will be held by the 2020 election as part of the agreement between the Green and Labour parties.
The Green Party says criminalising cannabis hasn't worked and a regulated market would enable more control over the drug.
Read more: Kiwis to vote on changing cannabis laws
But John Paul College principal Patrick Walsh said the prospect of teenagers knowing that cannabis may be lawful had sent "a collective shiver up the backbone of secondary school principals".
"Many of my colleagues are already fighting a rear-guard action to keep drugs out of school, particularly in communities where it is endemic.
"The simple fact is, like alcohol, whatever safeguards the government attempts to put in place to prevent teenagers from using it, legalising cannabis use will be equated with legitimising it.
"On this basis, it is highly probable more of it will find its way into schools being consumed or sold on back fields, toilet blocks and in student cars."
Mr Walsh said experienced principals would know the outcomes in terms of "lost educational opportunities, truancy, violence, depression and driver risks".
"Deans, counsellors and deputy principals can anticipate an increased workload and good students will miss out on critical teaching time while staff attend to drugged-out students.
"I know from my own experience working with teenagers that there is a strong link between cannabis, criminal offending, mental illness and poor academic achievement.
"We were promised many years ago that when the drinking age was reduced to 18 that regulation and education programmes for teenagers would prevent abuse - It is common knowledge that in 2017 teenage drinking remains a serious problem.
"High school students are prone to experimentation, risk taking and rule breaking. They are also our most precious resource and our future depends on them having a 'healthy mind and body' as they grow into adulthood.
"As an experienced secondary school principal, I simply ask all New Zealanders to think deeply on the wider implications of this proposed referendum, particularly on the young."
Rotorua Lakes High School principal Bruce Walker said he agreed the effects of cannabis on young people were "horrendous" but he was not opposed to the referendum.
"My personal position is that nobody under 20 should be using marijuana but it has become clear a referendum has to be done.
"I would be in favour of decriminalising cannabis if it meant gangs were not making a fortune off it.
"At the moment we have a zero tolerance for alcohol and cigarettes on school grounds so if cannabis was to be made legal, it would come under the same zero tolerance policy.
"Yes, there will be occasions when students bring it to school, but we will deal with that the same way we deal with prohibited substances now."
Greens leader James Shaw said legalising cannabis had been party policy for 20 years and the use of the substance should be a health issue, not a criminal justice issue.
"The prohibition model hasn't worked, plus it puts the entire trade into the hands of gangs.
"If you had a regulated market, the same way we do with alcohol and tobacco, you can control the price, advertising, point of sale, quality, and run full public health education campaigns."
Mr Shaw said public opinion on cannabis use had shifted, and a referendum on the issue by 2020 would be timely.