The Association of Teachers and Lecturers in Wales has described the PISA results as 'grim', but said changing policy could be perilous.
We should not pretend that this result is anything but bad news. While we have shown a modest improvement in our score for reading, the continued drop in maths results is very concerning and the dip in science results is deeply disturbing. There is still a great deal to do before Wales achieves the UK average, let alone becomes one of the top twenty nations.
However, given that the opening decade of devolution was characterised by denial, drift, and dither it would have been ludicrous to assume that our position could have been turned around in just two years.
– Dr. Philip Dixon, Director of ATL Cymru
Dr Dixon added: "Since [the last PISA results] the right policies and the right focus have been put in place. Literacy and numeracy have been made key priorities, and national frameworks and structures set up to support them.
"Those have not yet had time to make a real impact. They will, and we must give them time to work. The Pisa results are grim but we change course now at our peril."
The Institute of Directors has described Wales' PISA results as 'depressing', 'damning' and 'gravely concerning'.
Wales' results are bitterly disappointing, as a modern economy needs well-educated, numerate and literate individuals coming through its education system in order to gain the competitive advantage on the unforgiving global stage.
Countries with an unrelenting focus on the quality and rigour of its education system will be the ones who win that race.
Unfortunately, thanks to historic complacency, the UK appears to have its feet shackled at the starting line.
These figures quite clearly demonstrate that we are at a considerable disadvantage here in Wales and this causes us grave concern.
– Robert Lloyd Griffiths, Director Wales, Institute of Directors
He added: "Too frequently, impressive examination results have acted as a false barometer of actual attainment and competence.
"This deceit carries with it a heavy social and economic cost, as hundreds of thousands of school leavers seek to enter the workplace without the basic levels of literacy and numeracy.
"Radical decisions are needed to remedy this situation and to ensure improvements."
Education Minister Huw Lewis has responded to the publication of the PISA international education league tables:
Today’s results are disappointing and show we’ve still got a way to go before we close the gap with the OECD’s best performing countries. There are signs of some progress in reading, but significant improvement was never likely at this stage.
Everybody working in and around the Welsh Education sector needs to take a long hard look in the mirror this week. The PISA results are stark and the message is very clear, we must improve educational attainment and standards right across the board.
I am confident that the measures we’ve put in place since the last set of PISA results are the right way forward for Wales and we won’t be distracted from delivering them. Today’s news simply reinforces our case for the ambitious reforms we have already developed and everyone across the education sector in Wales now needs to play their part.
Through the new reading and numeracy tests, secondary school banding, extra funding for new schools and more rigorous qualifications system we are changing the way education is done in Wales, but it will take time to have a significant impact.
There are no quick fixes. I expect to see the impact of our reforms reflected in the next set of results. They’re ambitious and I believe they will have a lasting, sustainable and positive effect on education in Wales.
I hope this set of results strengthens the education sector in Wales’ resolve to improve our PISA performance in 2015. We owe young people in Wales nothing less than the best.
Professor David Egan, Professor of Welsh Education Policy, University of Wales Trinity Saint David, and a former adviser to the Welsh Government, said there are cultural reasons why Welsh pupils are struggling - particularly in terms of the strong relationship between poverty and attainment here.
"Yes, young people need to work harder, we need better quality teaching, local authorities and a Welsh Government that is more focused on those issues - but I think we also need to look at the social issues", he said.
"Quite clearly some of these other countries topping the PISA league tables that have as much disadvantage, as much poverty, as we do - seem to overcome that. Everybody buys into the idea that doing well has got to be a good thing to do."
Wales has fallen further behind in a worldwide assessment of pupils' maths, reading and science skills.
Welsh pupils are again behind their counterparts in England, Scotland and Northern Ireland in all three areas, and are below the global average in those areas.
Results from the so-called PISA (Programme for International Student Assessment) tests, sat by 15-year-old pupils around the world last year, have been published today by the Organisation for Economic Co-operation and Development.
Wales is now ranked 43rd out of 68 countries for maths, 41st for reading, and 36th for science - all lower positions than when the tests were last held in 2009.
The Welsh Government has previously targeted the top 20 positions of the influential league tables when the next tests are sat in 2015.
The latest PISA results, due to be released later this morning, focus on maths skills - the area where Welsh pupils particularly struggled in 2009.
PISA tests are designed to assess how students towards the end of compulsory education can apply their knowledge to real-life situations, rather than just a school curriculum.
There are concerns that young people leaving school in Wales are not well equipped to enter the workplace.
Pamela Simpson is Operations Manager for Markes International, a high-tech company which manufactures scientific equipment, in Llantrisant in Rhondda Cynon Taf.
She said: "it is a concern for all employers in this area - the level of numeracy and literacy in terms of school leavers."
"If you are going to fill jobs you need to have that pool of school leavers that are coming through that are educated to that sort of standard. It does make it a little bit harder if you don't have that background level of skills."
Wales' position in the international education rankings will be revealed this morning, when the results of the so-called PISA tests are published.
15-year-old pupils from more than 60 countries around the world were tested on their maths, reading and science skills.
The tests are held once every three years and when the results were last published, in December 2010, Welsh pupils scored lower than their counterparts in England, Scotland and Northern Ireland in all three areas.
In the worldwide tables, Wales was ranked 40th out of 67 countries for maths, 38th for reading, and 30th for science.
The Welsh Government described those results as "unacceptable" and "a wake-up call", launching a plan to improve standards, which included the banding of secondary schools and the launch of a literacy and numeracy framework.
It targeted a place in the world's top 20 when PISA tests are sat again in 2015.
Current Education Minister Huw Lewis has though warned not to expect a significant improvement this time.
PISA (Programme for International Student Assessment) is the world's biggest international education survey, held by the Organisation for Economic Co-operation and Development.
The latest results - from tests which pupils sat last year - are due to be published at 10am this morning.