International assessments, such as the Program for International Student Assessment (PISA), are being used to recommend educational policies to improve student achievement. This study shows that the cross-sectional estimates behind such recommendations may be biased. We use a unique data set from one country that applied the PISA mathematics test in 2012 in ninth grade to all students who had taken the Trends in International Mathematics and Science Survey (TIMSS) test in 2011 and collected information on students' teachers in ninth grade. These data allowed us to more precisely estimate the effects of classroom variables on students' PISA performance. Our results suggest that the positive roles of teacher "quality" and "opportunity to learn" in improving student performance are much more modest than claimed in PISA documents.