Teachers in Wales are bracing themselves for intense scrutiny and tough new accountability measures after the Pisa survey showed they had performed significantly worse than the other UK nations.
The Welsh Education Minister, Leighton Andrews, said there could be "no alibis and no excuses" for the "disappointing" performance.
In maths and reading, pupils' results were significantly below the OECD average, and Welsh 15-year-olds performed worse in all three subjects tested - reading, maths and science - than in 2006.
Mr Andrews said the results could not be "argued away or excused", adding: "Schools, local government and Government need to look honestly at these results and accept responsibility for them."
England's showing was broadly on a par with that of Scotland: reading - Scotland 500, England 495; maths - Scotland 499, England 493; science - Scotland 514, England 515.
Overall, the UK is ranked 25th for reading, 28th for maths and 16th for science.
Michael Gove, the Westminster Education Secretary, said his proposed reforms had been inspired by the Finnish education system. He was nevertheless "daunted" by the scale of the challenge England faced, he said.
"Other countries have been improving rapidly and, despite the massive investment over the last 13 years, we haven't been improving at the rate we should have been," Mr Gove commented.
The UK overall spends pound;54,000 per pupil, while Germany and Hungary achieve a similar performance for pound;40,000 and pound;28,000. Only seven other OECD countries spend more per student than the UK. Secondary school teachers in the UK are paid in line with these countries but work longer hours than the OECD average.