International studies like TIMSS (Trends in International Mathematics and Science Study) usually highlight students’ study result changes between two assessments. The aim of this study is to identify factors that contribute to the explanation why some schools is effective but others are less effective in terms of the students’ academic achievement in mathematics on TIMSS. We focus on the school efficiency in Sweden and Norway at the time points 2003, 2007 and 2011. In the core subject mathematics, Sweden’s result declined in TIMSS between 2003 and 2011 while Norway’s result inclined. Since Sweden and Norway have similar educational systems it is of interest to examine the incline in Norway as opposed to the decline in Sweden between 2003 and 2011. We used multilevel analysis in order to separate the effect of school level variables from the effect of student’s home environment and to take care of the sampling design used in TIMSS. The results show that Norway and Sweden exhibit different trends. In Norway it was possible to identify school level factors which affect the students’ achievements. In Sweden this could not be obtained. Note, in both countries at all three timepoints most of the student home background factors were significant suggesting that the students’ background play a large role in mathematics achievement.