Principals are worried by official predictions that schools will be short of 2450 teachers by 2025.
The Education Ministry has forecast the teacher shortage will ease for primary schools, but get a lot worse for secondary schools over the next eight years.
It estimated that without further action, the shortfall of secondary teachers would grow from 170 next year to 2210 by 2025, while the shortage of primary teachers would reduce from 650 next year to a surplus of 90 teachers by 2023 but worsen again after that date.
The forecast said the increase would drive up the demand for secondary teachers from 26,040 this year to 28,550 by 2025, an increase of 2500.
The ministry's Deputy Secretary of Early Learning and Student Achievement, Ellen MacGregor-Reid, said the shortage of secondary teachers would only happen if nobody took any action.
"Beyond 2020 for secondary schools we're not forecasting, it is a planning tool, so based on nothing happening we'd be looking for around 200 secondary teachers however we are confident with the measures we are taking now, that won't become a reality."
She said the measures being taken included more help for schools recruiting teachers from overseas, and one recruitment agency had received more than 1000 expressions of interest from teachers based in other countries.
Secondary Principals Association president Mike Williams said he did not share that confidence.
He said the ministry's forecast stated that the shortage would happen only if there were no changes to the current policy settings for recruiting and training teachers.
"That would imply if we're going to avoid being 2000 teachers short, we need some new policy settings... We've seen some more money on the same initiatives, but just putting more money in to the existing initiatives is not a new policy setting," he said.
"Their own modelling is showing some pretty serious shortages, 200 next year and growing every year after that and that's got to be very worrying. Something different's got to happen."
Mr Williams said the government needed to encourage more New Zealanders to become teachers and that required improvements to teachers' status in the community as well as their salaries and workloads.
The ministry's figures showed an improving situation for primary schools.
The ministry's forecast said the supply of primary school teachers would be 650 lower than demand in 2019 and the supply of secondary teachers would be 170 lower than demand next year.
However, Principals Federation president Whetu Cormick said he took no comfort from that forecast.
"We want an assurance that this data is actually correct. We were told just recently that by 2020 things would flatline and we wouldn't have the shortage, but now in the documentation coming out of the ministry they're talking of about 2023 things will flatline.
Mr Cormick said he doubted the government would be able to recruit the 900 extra teachers it was aiming to find for next year.
In Auckland, the mismatch between supply and demand would leave the city's primary schools short of 260 teachers next year, while its secondary schools would be short of 130 teachers.
The ministry said there were currently enough teachers to cover the positions it funded on the basis of each school's enrolments, but schools were choosing to employ more teachers than that using their own funds.
The percentage of teachers employed above schools' roll-based entitlements was 2.8 percent in primary schools and seven percent in secondary schools, it said.
Ms MacGregor-Reid said the ministry's projections beyond 2020 were speculative and should be treated with caution.
She said the number of teacher trainees was starting to increase, more former teachers were starting to return to the workforce, and the ministry was trying to attract more teachers from overseas.
"One of our recruiters has had over a thousand expressions of interest, either from Kiwis returning home, wanting to take up the support, or from well-qualified international teachers."
Ms MacGregor-Reid said there were enough teachers to meet the number of teachers funded by the ministry based on schools' class sizes, but some schools hired additional teachers from their own funds.
"If you were to take class size ratios, the 70,000 teachers we currently have active in New Zealand are more than enough to meet those class size ratios, but schools for a range of reasons, using what we call their operational funds, hire teachers above and beyond those class size ratios," she said.
"It is those additional teachers that add pressure into the demand, so around about three percent for primary and seven percent for secondary."
Ms MacGregor-Reid said principals were telling the ministry that Auckland and Tauranga were experiencing teacher shortages and areas such as Northland had also experienced supply pressures.
The forecast showed schools were employing hundreds of teachers over the age of 65 and some who were over 75.
The forecast projected primary school enrolments would stop increasing next year at more than 500,000 children, and the supply of primary teachers would meet demand by 2023. But enrolments in secondary schools would increase from about 2021 to exceed 300,000 students by 2024.
The ministry said its forecasting tool was based on birth rates, migration figures, and past trends for teachers leaving and entering the workforce.
It showed net migration of school-aged children had risen sharply from a net loss of about 2000 children in 2013 to a net gain of nearly 12,000 in 2018. However, the ministry forecast the net gain from migration would stabilise at more than 6000 children per year by about 2024.