Swedish schools have plunged from being one of the best in the world to average, according to a report published Tuesday.

The three-yearly Programme for International Student Assessment (PISA) report, which compares the knowledge and skills of 15-year-olds in countries around the world, ranked the Sweden 15th out of 34 Organisation for Economic Cooperation and Development (OECD) countries for reading skills, 20th for mathematics and at a below-average 23rd for mathematics.

"We differ from other countries because we have problems with equality (between schools and between students within the same school) and at the same time we have problems with our results in general," said Helen Aengmo, acting general director of the for Swedish National Agency for Education.

Swedish schools were traditionally among the top in global rankings, but have seen a constant drop in PISA results since 2000, with the weakest students dragging down the ranking.

In the 2000 PISA report, there was a smaller gap between the best and worst students, and between the best and worst schools, in Sweden than the OECD average.

In 2009, the gap between the best and worst Swedish students was larger than the OECD average, and while there was still less difference between good and bad schools than in many OECD countries, the gap in Sweden is now twice as wide as in 2000.