Huw Lewis cautions on Wales 2013 Pisa classroom results
Parents should not expect to see a rise in Wales' literacy, maths and science results in international rankings from next month, the education minister warned.
Wales was ranked lowest of the UK countries in the 2010 Pisa statistics.
First Minister Carwyn Jones has previously said that he expects to see Wales' Pisa results improve.
But Huw Lewis told BBC Wales: "I would not be surprised to see results which were, you know, far from inspiring."
Speaking to the BBC Radio Wales' Jason Mohammad programme, he said: "I've cautioned everyone involved.
"I wouldn't expect great improvement from these results that are coming out in December, because they relate to tests taken in 2012.
"Our school improvement plan was only just beginning to be implemented when those kids were taking those tests."
Mr Jones told the Senedd in October that he expected to see an improvement in Wales' Pisa results.
The first of them, for maths, is published in December.
He told AMs last month: "I've said that before, and if you see what we have done as a government to assist with the improvements to teaching in schools in Wales, I believe we are doing so."
Pisa assessments are published by the international body the Organisation for Economic Co-operation and Development (OECD).
The tests ranked Wales as the lowest country in the UK in December 2010 with Wales' teenagers below average, alongside the Czech Republic, in reading.
Out of 67 countries taking part, Wales was ranked 38th for reading, 40th for maths and 30th for the tests for science.
At the time the then-education minister Leighton Andrews called the results "unacceptable" and said everyone involved should be "alarmed".
The rankings led to changes being brought into the education system with the aim of getting Wales into the top 20 countries by 2015.
But last year, Mr Andrews - who has since left the cabinet to be replaced by Huw Lewis - was reported as saying that he did not expect the changes to deliver a "real improvement" in time for December.
Education expert David Reynolds, an adviser to the Welsh government, has also said that "nobody is expecting the Pisa 2013 results to be very different from what they were in the Pisa before".
Last year the Confederation of British Industry (CBI) called on the Welsh government to take urgent action over the state of education in Wales.
It followed a schools inspectors report which found 40% of children reached secondary school with a reading ability below their age.