ERO chief executive and chief review officer, Dr Graham Stoop, said schools that were identified as partially effective needed to use their assessment information to design a mathematics curriculum that worked for all students.
"By taking a more confident and integrated approach, this large group of schools could move to having highly effective curriculum review and design processes. This would make such a difference, particularly for students who are currently achieving below the National Standards," he said.
"Schools have permission to be flexible and change their mathematics curriculum to respond to the specific needs of students, particularly those considered priority learners."
Meanwhile, a Wellington principal said the report showed that National Standards had already begun to negatively impact on children's learning.
Louise Green, of Khandallah Primary, said National Standards were "an inflexible tool" and resulted in a system where educators were being forced to teach-to-test.
"Teachers have been concerned for some time that the result of testing and measuring required by National Standards would result in inflexibility in the curriculum. Now we have some proof," she said.
"This is proof that National Standards is a policy that will damage our education system. It does not foster a climate where teachers can be flexible and creative and spend time with children who are struggling."
She said better resourcing was needed so teachers could spend more time with children who were struggling.
The ERO report made several recommendations for schools and the Ministry of Education to address the problem, Dr Stoop said.
