Wales's 15-year-olds were disappointing in a 2006 global assessment of reading, maths and science because they were brought up on a testing regime, according to the president of the Association of School and College Leaders.
Brian Lightman, headteacher of St Cyres School in Penarth, defended the nation's poor rankings in the Programme for International Student Assessment (Pisa) to delegates at the union's annual conference in Brighton last week.
He predicted that Wales's results in Pisa would improve greatly under a new system of teacher assessment.
The results, published last December by the Organisation for Economic Co-operation and Development, left Wales ranked 28 in reading and 34 in maths, tailing the home nations and placing Wales on a par with Croatia.
It was the first year Wales had taken part in the assessment.
Experts said the country had done badly because of the poor performance of its brightest pupils. But in an attack on England's league-table culture, Mr Lightman said Wales lagged behind because participating pupils had undergone national tests at seven, 11 and 14, something England is continuing to do but which Wales has abolished.
Quoting Professor Richard Daugherty from the University of Wales Aberystwyth, who led the 2003 review of national curriculum assessment in Wales, Mr Lightman said the poor Pisa results were down to a "test-ridden system of schooling".
Schools and colleges in Wales, he said, now knew themselves better than ever, a consequence of their own "fiercely self-critical" evaluation, whereby schools would go the extra mile to raise standards.
"That ethos cannot develop within a culture of blame in which an over-inflated accountability regime constantly forces us to look over shoulders," he said.
A full breakdown of Wales's Pisa results was posted online by the Assembly government this week. The summary shows schools' participation fell slightly below the expected level of 85 per cent.
While scores for science results - the main focus of the 2006 assessment - were average compared with other counties, both maths and reading were below average. Boys from Wales who took the tests did significantly better than girls in science and maths. Girls recorded higher marks in reading.
ASCL conference, pages 22-23.