Schools could get as many as 21 extra teachers under Labour's election-year policy, according to new calculations.
Larger schools are more likely to have bigger class sizes and would be the biggest beneficiaries of the proposed changes.
Labour's class size policy was the centrepiece of leader David Cunliffe's address to about 1000 members and delegates at Labour's election year Congress on Sunday. It promises to fund one teacher to 26 students at primary schools from 2018, and introduce a maximum average class size of 23 at secondary schools.
Using recent roll information the Post Primary Teachers' Association (PPTA) has calculated how some schools could be affected. It says large schools - many of which are in the Auckland region - are in line for the biggest increases in teacher numbers.
Rangitoto College, with a roll of 2821, would get almost 21 new teachers under the policy by 2018, the union says.
Mt Albert Grammar School would get almost 19, while nearby 2447-strong Auckland Grammar School would be boosted by nearly 15 new teachers.
"The current staffing formula disadvantages large schools like ours," said MAGS headmaster Dale Burden. "[Labour's policy] should mean that the bigger schools aren't disadvantaged. So that's a positive."
The class sizes policy together with other announcements in the past week, including paying schools that do not ask for donations, will cost about $850 million over four years.
Mr Burden said for secondary schools it would not be as straightforward as reducing class sizes across all subjects.
Read more on Labour's education policy here.
"We might have a big French class, but we're not going to have an entire [new] French teacher just to pick-up smaller French classes."
Mr Burden - like the PPTA - is also a supporter of the Government's $359 million programme that is focused on using the best principals and teachers to spread good practice. Labour and the NZEI have said it is unlikely to foster collaboration between schools.
National, for its part, has questioned the true cost of Labour's education policies and says investing in the quality of all teachers is more important than class sizes. It also points out that it currently funds secondary schools for an average of 20 students per classroom.
However, because resourcing ratios do not include the time during the school day that teachers are not in contact with students, actual class sizes can be bigger.
Mr Burden told the Herald that education was too important to act as a political football each election cycle and in an ideal world aspects of both plans could be locked-in long term.