Schools are increasingly using their own funds to hire teachers over and above those paid for centrally by the Government, new figures reveal.
The number of "extra" teachers paid by schools has shot up by more than 50 per cent over the past four years.
A principals' group suspects some schools are increasingly using income from international students to hire the extra teachers and keep class sizes down and subjects on offer.
At Auckland's Rangitoto College, principal David Hodge said fees from international students cover 40 per cent of the school's day-to-day running costs and allows it to hire 17 extra teachers.
"High decile schools would have to dramatically cut services if there was any major downturn in the [international student] market," Hodge told the Herald.
"So much so that the schools would almost be unrecognisable."
The Ministry of Education tells schools how many teachers it will allocate based on the roll, with associated salaries paid centrally.
Schools can employ more teachers but need to use locally-raised funds or school operations grants, which are expected to cover day-to-day operation costs.
Full time equivalent teacher numbers funded in this way by state and state-integrated secondary schools was fairly steady since 2008, but increased from 1110 in 2013 to 1717 this year - a 54 per cent jump.
The Green Party, which secured the teaching figures through a Parliamentary question, says not all schools can attract international students or raise funds.
Its education spokeswoman Catherine Delahunty said she knew of principals that regularly attended education fairs overseas to tout for students in order to keep class sizes down.
"The Government needs to fund the education system so that international students are a nice to have, rather than a way that you can keep your class sizes and teaching staff to reasonable levels."
Secondary Principals Council chair Allan Vester said he suspected the figures showed that schools that had international student income were using it to employ more teachers.
That was done to maintain or reduce class sizes, and also keep offering more borderline subjects.
Schools' ability to do that varied hugely, and that created an issue around equity, Vester said. Another concern was what would happen if there was a downturn in the international education market.
Hodge said Rangitoto had put aside a substantial contingency fund to allow it to cope with any short-term downturn, and also draw students from a variety of markets - its 240 full time equivalent international students come from 30 different countries.
As well as receiving less money as a decile 10 school, Hodge said large schools such as Rangitoto were disadvantaged by the staffing formula used to determine how many teachers the ministry would pay for.
A ministry and PPTA review into school staffing in 2012 made recommendations including that the formula be reviewed in relation to larger schools. That has not happened.
Between 2013 and 2015, international student enrolments in New Zealand secondary schools rose by about 7 per cent to 15,408.
Kim Shannon, the ministry's head of sector enablement and support, said that increase could be one reason for the figures cited by the Green Party.
Another was because schools had more discretionary funding, with spending on state and state-integrated schools rising by 34 per cent between 2008 and 2015.
New Zealand's child-teacher ratios are among the lowest in the OECD, Shannon said, but schools had the flexibility to use their operations grants as they saw fit.
"The vast majority of our schools deliver our world-beating curriculum within their budgets and do it well."
Acting Education Minister Anne Tolley said the Government had an "undeniable" record of increasing education spending.
"The $11 billion we are spending is the biggest investment in education and early childhood education this country has ever seen."
"Extra" teachers paid for by secondary schools to supplement those centrally paid by the Ministry of Education.
Source: Parliamentary question response to Green Party. Figures are full time teacher equivalents funded from operational funding (including self-generated funds) for state and state-integrated schools.