The OECD says not too much should be read into schools' national standards results.
A report published on Thursday says the number of students above and below the national standards provides little insight into the quality of a school.
Radio New Zealand's education correspondent says the agency's warning goes to the heart of one of the main criticisms of national standards - that schools will be judged on their results and compared with others in league tables.
Schools are expected to provide their results for 2011 to the Ministry of Education shortly.
The OECD report - based on a visit in 2010 - says it is of utmost importance to clarify what kind of information the standards can and cannot provide, who should have access to the information and what it can be used for.
The report also calls for more work to ensure teachers at different schools are making similar judgements.