The Pisa tests also showed that Swedish students’ declining performance in science had been reversed.
Singapore topped the rankings of the 72 countries, followed by Japan and Estonia. Sweden, meanwhile, was 28th, up from 38th when the test results were presented three years ago.
More than half a million 15-year-olds took the two-hour test in 2015 and were assessed in science, mathematics and reading.
Results showed that Swedish students scored above the OECD average in reading and around average in science and mathematics.
The OECD had called for school reform in Sweden following the results of the 2012 tests in which Sweden went from average to significantly below average over a decade. It had been the steepest decline of any country which takes part in the tests.
Despite the improvements this time around, there were still some negatives. There were growing inequalities in the outcomes for advantaged and disadvantaged students. The gap between those groups is now wider than the OECD average.
There is also a wider than average gap between the highest and lowest performing students. Indeed, the difference between the top 10% and the bottom 10% of students has increased by 27 points since 2006.
The report also said Sweden was “facing a difficult challenge with immigrant students”. The share of first and second generation immigrant students is growing, with the gap in science results between immigrant and non-immigrant students now wider than the OECD average.