The study looked at maths, English and science

The Organisation for Economic Co-operation and Development (OECD) assesses results every three years.

The Programme for International Student Assessment (Pisa) looks at the performance of 15-year-olds.

The Scottish Government said it had inherited a challenge which it was determined to tackle.

Research, which was carried out in 101 schools and involved about 2,700 students, took place in March last year.

The report suggested that four countries were better than Scotland at science, eight were better at maths and five were better in reading.

A few other countries are doing better faster Maureen Watt Minister for Skills and Schools

Three years ago only three countries performed better than Scotland in all three subjects.

Minister for Skills and Schools, Maureen Watt, said: "While it is clear that many of our pupils are still performing above the OECD average, we have inherited a situation which shows our international ranking has fallen since the last survey in 2003.

"A few other countries are doing better faster. The study also shows that the impact of a disadvantaged background has a greater effect on how our students perform than many other countries.

"That's why this government is investing in early intervention to ensure our children get the help and support they need to spot and deal with problems before they start having an impact.

"And we are developing the Curriculum for Excellence which will improve standards for all children in our schools."

She added: "This, taken with the information from the recent international literacy study and the forthcoming OECD review of school education in Scotland, provides us with valuable insights into our strengths and weaknesses which we will use to ensure all our young people can take advantage of the opportunities available."