Christchurch's Cashmere High school will be reducing its zone at the start of the 2019 academic year.

A zoning change at a Christchurch school means some students living just 800 metres away will now have to go elsewhere, while others living more than 40 kilometres away are welcome.

Cashmere High School's new zone, which comes into effect next year, will exclude those living northwest of Stourbridge St, Barrington St, Coronation St and Brougham St and east of Opawa Rd. It has has left some nearby residents frustrated and upset they cannot access their closest school.

Board of trustees chairman Geordie Hooft said the Ministry of Education required the school to drop its roll by about 308 students.

CASHMERE BOARD OF TRUSTEES/SUPPLIED A map of the zone changes to be incorporated at the beginning of the 2019 academic year.

Any change to the school's zone would disappoint some people, but the boundary had to be reduced, he said.

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Somerfield Residents' Association chairwoman Julie Tobbell, who has a child that attends Cashmere, said some students living just outside the new zone would exchange a 1.4km walk to Cashmere for a 2.5km one to the next closest school – Hillmorton High.

GEORGE HEARD/STUFF Students on Therese St, just 800 metres from Cashmere High School, are in the zone for Hillmorton High School.

"It seems a little bit illogical that students will live close and not be allowed to go," she said.

"There's been quite a few people really quite concerned about the impact the move will have on the families and their ability to go to school."

A resident on Therese St, which is just 800m from Cashmere, said children should have the choice to attend their closest local high school.

Cashmere High School principal Mark Wilson.

The woman, who asked to remain anonymous, said it was unfair students living 41.3km away in Port Levy could attend Cashmere, but those living in her street and nearby would miss the cut.

The Government needed to increase funding and improve infrastructure for the other schools in the area, like Hillmorton High, if they were to provide the same quality of schooling as Cashmere, she said.

"There will always be a line and others on the other side will be affected and unhappy. I am just disappointed about how they went about this."

She acknowledged the school, which had about 2008 students, had become too big, but said efforts to reduce the roll should not come at expense of locals living less than 1km away.

Hooft said the school's steady roll rise over the past few years could be attributed to the work of principal Mark Wilson.

"We are in some ways a victim of our own success," he said.

Hillmorton High principal Ann Brokenshire said it was clear some of her school's buildings were old, but they would be demolished once a new middle school with 22 classrooms was built next year.

Twenty of the buildings will be owned by Hillmorton and two will become a satellite unit for Van Asch Deaf education centre.

She noted other Christchurch schools, including Cashmere, had already received infrastructure improvements as part of the Ministry of Education's Christchurch Rebuild Programme, and said Hillmorton had been negotiating with the ministry regarding its rebuild and school roll sizes.

"It has been a very thorough process, where we are trying to make sure that people understand that the biggest difference in schools in Christchurch are within schools not across schools."

Hillmorton's next challenge was planning for a roll increase while maintaining its unique culture, she said.

"What we like is the school is relatively small [with 780 students] and the students describe it as being whānau-like. We want to protect that but we know we are going to get bigger."