Lower-decile schools have more teachers per student than their higher-decile counterparts.
In statistics released from think-tank The New Zealand Initiative yesterday, compiled from the Ministry of Education, decile 10 schools had an average of 24.6 students per teacher to 20.6 for decile 1 schools.
The student-teacher ratio is an important indicator of the level of resources devoted to education, according to the OECD.
Rose Patterson, research fellow at The New Zealand Initiative, said that although parents believed smaller class sizes would benefit their child, the enrolment trends at higher-decile schools showed they had other priorities.
As parents sought a high-quality education for their children, they drove the class sizes up in high-decile schools - as their rating was often seen as a proxy for quality.
"The irony of this is that as the student population migrates towards higher-decile schools, this changes the number of teachers schools are entitled to," Patterson said.
"In 2004 the proportion of students in decile 1 to 5 and 6 to 10 schools was roughly half and half; by 2012, 60 per cent of students were at decile 6 to 10 schools, and 40 per cent at decile 1 to 4."
But those who work in lower-decile schools still fiercely advocate for small class size, believing their students benefit from extra attention in the classroom.