* Scotland scored above the international average in another survey - the Trends in Mathematics and Science Study (Timms) - but came out less well than in the Pisa results (see right).
Timms measures maths and science performance at P5 and S2 (nine and 13-year-olds) and the results put Scotland further down the league table, although still above the international average.
Scotland was slightly below the international average in maths at P5 but significantly higher by S2; in science, Scotland's performance was above the international average at both P5 and S2.
However, at the S2 stage of maths, Scotland was placed behind 17 other education systems. At the P5 stage of maths, Scotland was 18th. In science at S2, Scotland was ranked 19th and 16th at P5.
Although the Timms rankings may disappoint the Executive, the report shows that Scottish results in science at P5, although still marginally below the international average, have improved significantly since 1995, when the survey was last carried out.
There was negligible difference in many countries between the results from 13-year-old boys and girls doing maths. But at nine, boys did better in five countries, including Scotland. Boys also outperformed girls in P5 science in Scotland and three other countries.
The Scottish Executive has noted there is an important difference in the nature of approaches taken by Pisa and Timms, which is run by the International Association for the Evaluation of Educational Achievement.
It says the assessments used in Timms were constructed on the basis of an analysis of the intended curriculum in each participating country. By comparison, Pisa looks at students' ability to apply their skills in real-life situations.
Timms also assesses students at a younger age (13) compared to Pisa (15), when pupils are preparing to finish the compulsory phase of their schooling.