The primary school teachers' union says many schools are at breaking point and changes in the way schools are funded will only worsen the problem.
At the NZ Educational Institute's annual conference, the union's president Louise Green presented the union's "report card" on the Government, scoring it "well below standard".
She said the OECD Education at a Glance report showed New Zealand spent $10,151 per student compared to the OECD average of $11,701.
"It is an embarrassment for a wealthy country like ours that primary schools receive so little money compared to other countries."
A spokeswoman for Acting Education Minister Anne Tolley said New Zealand spent a higher percentage of its public funding on education than any other OECD country and the amount spent had increased by 35 per cent since 2009 to 2011.
That included more than doubling the amount spent on early childhood education, a 39 per cent increase in primary schools since 2008 and 24 per cent in secondary schools.
A freeze on many schools' operations funding grants in 2017 meant some were slipping even further behind, Green said.
In the Budget, Education Minister Hekia Parata announced a $12.3 million increase for overall schools operations funding would be allocated according to the number of "at risk" students a school had, rather than spread across all schools.
Usually operations funding, which pays for the running costs and support staff at a school, increases by the rate of inflation.
Green said the new approach amounted to a freeze for many schools and a 2.3 per cent cut to the real value of per-student funding.
Parata has previously denied it was a freeze.
About 133,00 students had been identified as "at risk" and 99 per cent of schools would get an increase ranging from a few hundred dollars up to $57,000. One school was getting $109,000 more.
However, schools have argued that for many the increases are not enough to keep up with inflation in school running costs.
Labour leader Andrew Little said the issue of education funding was becoming a big issue and the targeted-funding approach was opaque.
"The operations funding has been frozen with a mysterious discretionary fund for the minister to give top-up funding, but nobody seems to know what the criteria are or how you get it."
Green also reiterated concerns with a Government proposal for "global budget" for schools, which unions claimed was bulk funding of salaries in disguise.
Although the Government is yet to decide whether to go further with that, teachers rejected it almost unanimously after stop-work meetings this month and warned of industrial action if the Government went ahead.
It was the only one of seven proposals from the Government's Education Funding Review to be rejected by an advisory group of educationalists set up by Parata.
Other proposals included scrapping the decile system and targeting funding more at at-risk students.
Any replacement funding system would not begin until 2019 at the earliest.
"Global funding" would include cash payments for schools' running costs and a "credit" arrangement for teachers' salaries. Unused credit would be paid out at the end of the year and principals could decide the split between cash and salary credits.
Teacher unions say it could result in a push to drive down spending on teacher salaries.