This is a useless study.  It compares government school systems to each other, based on arbitrary metrics comparing scores between groups of 15 year olds (an arbitrary age) in testing for arbitrarily chosen subjects.  It would be trivial for any number of different private school systems in the US to crush Finland's numbers.  Average scores for US students in government sponsored schools in the US suck, in part, because everyone knows that they suck so many parents with means don't send their children to government schools, so those government schools suck more.  Compare Finland's scores to say, Covington Latin in Covington, Kentucky or St Francis High School in Downtown Louisville, Kentucky or Highlands Latin School in the Highlands district of Louisville, Kentucky and these scores would be embarressing.  And those three are just a few that I could think of off the top of my head.  For that matter, just comparing Finland's average scoring to the average American homeschooled high school aged student would make a lot of Finnish schoolteachers question the effectiveness of their careers considering what the average, untrained parent educator can do.