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The report, Mathematics in Years 4 to 8: Developing a Responsive Curriculum, said about 50% of primary schools could make a considerable difference to their pupils' maths learning if they adjusted their curriculum to better meet children's needs.
These schools were viewed as being ''partially effective''.
The report was based on reviews of 240 primary schools and looked at what they were doing to raise the mathematics achievement of pupils in years 4 to 8.
ERO chief executive and chief review officer Dr Graham Stoop said the next step for ''partially effective'' schools was to use their assessment information to design a mathematics curriculum that worked for all pupils.
''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.''
ERO found while most schools were very good at identifying pupils needing more help in maths, they continued using the same teaching strategies and programmes which they had tried before.
Few schools had evidence these approaches actually accelerated the progress of struggling pupils.
Otago Primary Principals' Association immediate past president Brent Caldwell said it might be a sign National Standards had already started to negatively impact on children's learning.
Ensuring classroom programmes were tailored to meet the individual needs of pupils was a priority for all teachers, he said.
''The use of assessment information to identify priority learners and devising new and innovative strategies to assist them is part and parcel of every teaching team's work.
''The ERO report quite rightly indicates that accelerating the progress of students working below or well below the mathematics standards is challenging for most schools.
''What is needed is less emphasis on grading children against flawed standards and more resourcing put into support for teachers and schools from specialist advisers.
''The Government's focus on accountability via National Standards is a poor substitute for a proactive supportive service that could be used to assist teachers and schools.''