The study was not officially scheduled for publication until next Tuesday but a Spanish magazine has jumped the gun and released some of the results on its website.
The leaked results do not make happy reading for Swedish educationalists, with Sweden tumbling from 18th to 22nd place since the last PISA study in 2003.
The current study was based on tests carried out in 2006 on pupils in 57 countries.
Finland topped the rankings, strengthening its position at the top of the class in science subjects.
Hong Kong, Canada and Taiwan made up the rest of the top four.
Like Sweden, however, Denmark and Norway also fell down the rankings. Denmark dropped four places to 24th, while Norway plunged six places to 33rd.
The PISA report was the second major educational study to be released this week.
On Wednesday, Sweden learned that it had come tenth in a comparative study of reading skills among 10-year-olds in 45 countries and regions.
A total of 4,000 Swedish pupils from 150 schools were tested as part of the Progress in International Reading Literacy Study. The Russian Federation, Hong Kong and Alberta, Canda topped the literacy league.