Wellington's biggest private school, Scots College, has decided to admit girls across its whole age range from Years 1 to 13.
The change, announced today, follows similar decisions by St Peter's School in Cambridge in 1987, St Andrew's College in Christchurch in 2001 and St Kentigern College in Auckland in 2003.
King's College in Auckland admitted girls from Year 12 in 1980 and from Year 11 in 2016. St Paul's Collegiate in Hamilton admitted girls from Year 12 in 1985 and from Year 11 in 2010.
There will now be no boys-only school among the country's biggest 15 private schools. The biggest boys-only private school will now be Remuera primary school King's School, with 674 boys, followed by Christ's College in Christchurch with 657 boys.
All of the six originally boys-only schools in the biggest 15 have now admitted girls, but none of the four girls' schools in the big 15 have admitted boys.
Scots College and its sister school Queen Margaret College were established by the Presbyterian Church in 1916 and 1919 respectively.
Scots' roll last year was 866 boys, making it the country's 11th biggest private school. Queen Margaret had 683 girls, the 15th largest private school.
Scots enrolled girls for the first time in Years 11 to 13 this year, making 60 places available for girls and actually enrolling 53.
Headmaster Graeme Yule said the school would now admit girls from next year in the "prep school" (Years 1 to 6) and at the start of the middle school (Year 7) and college (Year 9).
"The world is co-ed, attitudes towards gender-based roles or subjects are changing," he said.
"At the core of this decision is our belief that co-education across the college further enhances our values. We will continue to provide students with an all-round education that allows them to think critically, communicate effectively and develop the necessary skills to learn for life.
"In short, Scots College believes the move to co-education best prepares students for their future."
As at St Kentigern College, boys and girls will be educated in separate single-gender classes in the middle school (Years 7 to 10), but will be in co-education classes in Years 1 to 6 and again in Years 11 to 13.
"We believe this model, particularly during these initial years of co-education, will provide these early adolescent students with a transitional pathway combining the academic benefits with the social advantages," the college says.
The school has told parents on its website that the construction of a new block named after former headmaster Ian McKinnon has created space to expand the roll.
"The number of spaces available to girls is limited to ensure a managed transition," it says.
This allows us to grow the roll gradually while maintaining the same personalised education for all students as we develop the required teaching and learning facilities for a growing roll.
"It is clear from surveys we have undertaken in the past few years and in the consultation process that small class sizes has consistently been one of the main reasons for choosing Scots College. This is why the board have resolved to set specific class size limits. In Years 1 and 2 classes will have a maximum number of 15, in Year 3 of 20 and in the senior syndicate years 4-6 of 22 in each class.
"Once we reach the class size limit, a waitlist will be developed and at some year levels if numbers allow we will consider opening additional classes."