International test results are being used by the Welsh Government to introduce "seriously flawed" policies and as an excuse to "insult" the school workforce, according to a campaign launched by a teaching union.
The NASUWT has targeted all 60 Assembly Members and 22 local authority directors of education in Wales, urging them to take a "balanced and objective" view of the results of the 2009 Programme for International Student Assessment (Pisa) tests.
It claims that education minister Leighton Andrews is creating a crisis over Pisa to introduce policies and initiatives that will "undermine" the education system.
Wales's poor performance in the tests, run by the Organisation for Economic Co-operation and Development (OECD), was described as "disastrous" and "alarming" when the results were revealed last December.
The country's 15-year-olds fell further behind their counterparts in the rest of the UK in English, maths and science compared with the previous tests of 2006.
It prompted the education minister to launch a 20-point action plan in February to tackle underperformance, including the introduction of national reading tests, a school standards unit and a school banding system.
In its letter, the NASUWT says: "The way in which the Welsh Government has interpreted the outcomes of Pisa is seriously flawed and is leading to poorly thought-out lurches in policy that will damage rather than enhance the quality of educational provision in Wales."
It says the Pisa test results are one of a number of indicators a country can use, but on their own they are too small to either praise or criticise an entire system.
"It is quite right to use the rich data from Pisa to evaluate education systems," it says. "There are important lessons to be learned from other countries and evaluation is important. However, the data must be used appropriately to inform, not distorted to provide a pretext to justify any change."
However, one of Wales's leading educationalists dismissed the letter as "drivel".
David Reynolds, professor of education at Southampton University and a policy adviser to the Welsh Government, said: "The Pisa tests were developed by the best in the world and everyone accepts they are the new benchmark.
"The interpretation isn't flawed. The changes in the Welsh scores in the last three years can only reflect the state of the education system."
A Welsh Government spokesman said Pisa was a well-established and internationally respected test.
"We cannot ignore the fact that Wales is underperforming for all ability levels; it is not just Pisa that provides evidence of this. We can and must do better for our young people."