A new report into the staffing allocation models for secondary schools highlights the difficulties larger schools have in maintaining workable class sizes.
It comes from the Secondary Schools' Staffing Group, which was established to address issues surrounding class size.
It's highlighted concerns about larger schools not having the same opportunities to keep their class sizes at 25 or below.
Principal of Auckland's Rangitoto College David Hodge says the formula is deliberately designed to penalise larger schools.
He says it's because of the theory large schools have economies of scale.
"They could therefore always provide the numbers of students to fit the courses, unfortunately that's not something that's a reality, the theory doesn't always fit the reality."
Mr Hodge says there has to be some kind of formula but it needs to be realistic and based on what needs to be achieved.
PPTA junior vice-president Angela Roberts says the formula used to calculate staff funding no longer fits with 21st century schools.
She says it puts pressure on the ability of larger schools to maintain workable class sizes.
"The larger a school gets, basically once you hit 1,000 pupils, the more difficult it is for a school to have smaller classes if they choose, so the formula itself is a fundamental flaw in regard to larger schools."
Ms Roberts says there are fundamental flaws with the formula which need tweaking.
"That would require staffing to be put in and targeted to those particular schools, it's absolutely possible to identify which those schools are and how much staffing they would require."
She says they're looking forward to working with the Ministry of Education on finding solutions to the issue.