A Ministerial Advisory Group will also be set up to look at further changes to class sizes.
Education Minister Jan Tinetti announced the policy alongside Prime Minister Chris Hipkins at Remuera Intermediate School in Auckland this morning.
It would see classes in Year 4 to Year 8 - right up until high school - shift from 29 students per teacher to 28 students per teacher by 2025, at a cost of $106 million over five years.
This ratio is higher than for younger years, with one teacher per 15 students at Year 1.
This would be supported by an increase in teacher numbers, with 320 more full-time teachers the government says are in line to start from next year.
Tinetti said research had shown Years 4 to 8 were critical for children's learning.
The 2019 National Monitoring Study of Student Achievement found 63 percent of students were achieving at the expected level in Year 4 in writing, dropping to only 35 percent achieving as expected by Year 8. For reading, the numbers were 63 percent at Year 4 dropping to 56 percent by Year 8.
"This is often where maths and literacy achievement can begin falling behind. That's why we are targeting these years with more teaching resource," Tinetti said.
"I'm not happy with the downward trends we are seeing in maths, reading and writing. More teachers, targeted to where they are most needed is a practical way we can improve results for our kids."
She said teacher salaries had increased by 18 percent since Labour took office in 2017, and put initiatives in place to increase primary and intermediate teacher numbers by more than 3000.
This included a $24m investment last year focusing on teacher recruitment, aiming to attract and train a further 1000 teachers.
Tinetti said the ministerial advisory group made up of experts from the education sector would look into class sizes in the long term. This would include looking at school funding mechanisms and staffing entitlements.
"I want to get a deeper understanding of the key areas where change is most needed," she said.
"I want this work to happen fast so I've asked for the terms of reference to look at the challenges our teachers and students currently face in the classroom, what our school leadership and management need, what any further decrease to class sizes will cost and whether it is achievable."
