More than 3000 educators turned up for a meeting to discuss the Government's proposed change to how schools are funded.
The meeting at the Vodafone Events Centre in South Auckland capped a fortnight of events by NZEI Te Riu Roa and PPTA members on the issue.
"The Government's plans could see larger class sizes, less money for public schools, fewer teachers and narrower curriculum options," said PPTA president Angela Roberts.
"Educators have strong views on these issues and have been turning out in droves to have their say."
Education Minister Hekia Parata had said in an earlier statement that the review discussions were at a "very early stage".
She said nothing had been decided and changes won't be implemented until 2020.
Schools are currently funded based on a teacher-pupil ratio: one teacher for every 15 pupils at Year 1, and up to 29 students in Years 4 to 6.
If Parata's proposed funding change goes ahead, then these ratios would be replaced with flat funding per pupil.
Roberts said it would force schools to accept unprecedented numbers of students in their classes.
"Parents and our school communities trust the views of their children's educators," she said.
"It is critical the voice of those closest to children's learning is heard in this debate."
PPTA general secretary Michael Stevenson said it was the first time the teachers unions NZEI and the PPTA are joining forces following the Government's announcement of the proposed "global budget".
The budget would cover teaching and learning, including credits for teacher salaries.
Educators are worried that schools and their boards would lose the certainty of getting the staffing to match the number of students.
"It's going to allow the Government to push down all the hard decisions that they currently make on to individual schools," Stevenson said.
"In particular the ability to cash up teachers' salaries and enforce schools to use the money in other ways, for example property and maintenance."
"From Kaitaia to Stewart Island and across to the Chathams; principals, teachers, support staff, early childhood educators and Ministry of Education staff have turned out in record numbers to have their voice heard on the Government's radical plan for change," said NZEI president Louise Green.