Scottish education appears to be standing still despite significant spending and reforms over the past 10 years.
The picture emerges from an international report assessing children's skills in maths, science and reading at age 15, just before they are able to leave school.
The report, by the Organisation for Economic Co-operation and Development (OECD) studies education in 70 countries which together make up 90% of the world's economy.
Michael Davidson, a senior education analyst for the OECD, said: "Scotland seems to be treading water.
"Its performance is around average except in science where it is above average. The question is - is average good enough?"
He added: "Countries that have made advances have tackled issues vigorously.
"In Scotland around 16% of 15 year olds are struggling in reading which we know means that they are likely to always struggle in life.
"It is a huge mistake to assume that children from disadvantaged backgrounds are destined to fail."
The report, called the Programme for International Assessment (Pisa) indicates that children from less well-off backgrounds in Scotland do not do as well as they might in the education systems of countries such as Turkey, Canada and Finland.
Researchers found that Poland, Germany and Chile were among the countries that have brought in reforms which have produced tangible improvements.
Success seems to be linked to measures such as changes to teacher training, publication of a school's results, fewer constraints for frontline staff and early identification and support for children in danger of falling behind.
Success also seems to be linked to good pay for teachers but not to small classes.
The report indicates that mean maths scores dropped by 18 points between 2003 and 2006 and have now fallen by another seven points to 499.
Scores in reading are steady at 500, although 26 points below the level in 2000.
In science, scores have been steady at 514 points over the past 10 years and are now in line with the UK average.
Scotland is above the UK average in reading and maths.
Mike Russell, the education secretary, said the figures showed that the previous slide in performance had been halted.
He said: "Coming less than a week after new statistics showed that we are cutting class sizes, slowing the decline in teacher numbers and helping a greater proportion of school leavers into employment, training or education, the PISA results provide further cast-iron proof that for Scottish education, the tide has turned.
"They also disprove the inaccurate myth peddled by some that Scotland is lagging behind educational performance south of the border.
"Clearly that is not the case and Scotland - like England and Northern Ireland - is performing in line with international standards."
The OECD study indicates GDP spending per capita only explains 6% of the difference in pupil performance around the world.
A country's education policies are far more important, it suggests, pointing to the "stunning success" of Shanghai in China with "moderate" spending on children from a wide range of home backgrounds.
In the Pisa exam, high marks are not given for simply memorising and regurgitating information.
It measures a child's ability to interpret text, graphs and figures and solve problems.
These skills are seen as vitally important in the fast-moving 21st century when children will move into jobs and technology which have not yet been created.
Teaching union the EIS welcomed the study's findings.
General secretary Ronnie Smith said: "Overall, this shows a steady, solid performance by Scottish students.
"It is clear, however, that much needs to be done if Scotland is to maintain its international standing as other countries are prioritising and strengthening their education systems year-on-year."
'Unforgiving of frailty'
Mr Smith said that if resourced and supported, the Curriculum for Excellence (CfE) should enhance the performance of the Scottish education system.
He added: "Today's PISA report clearly acknowledges the link between high pupil performance and properly paid teachers.
"If Scotland is to progress, it is essential for the Scottish government and local authorities to ensure that their teachers are paid appropriately for the job they do, especially at a time of major curricular change."
In a foreword, the study highlights changes by countries such as Poland and Germany after disappointing results in previous Pisa surveys.
Urging similar reforms, it says: "The world is indifferent to tradition and past reputations, unforgiving of frailty and complacency and ignorant of custom and practice."
A report last year from the Centre for Public Policy for Regions, concluded spending on education is 20% higher in Scotland than in England but that this hasn't resulted in higher academic results.
Critics said higher spending was down to deprivation and a population spread which meant higher numbers in small village and island schools.