International assessments of mathematics have shown persistent and widely intensified socioeconomic inequalities in achievement worldwide over the last decades. Such achievement gaps may partly be due to the differences in students' personal and family characteristics. They may also be attributed to the schooling itself if school systems provide differentiated opportunity to learn (OTL) for children from privileged versus disadvantaged backgrounds. Previous research on the mechanism of the joint relationship among socioeconomic status (SES), academic achievement, and OTL produced inconclusive results. The main aim of the present study is to test whether schooling actually perpetuates social inequality in achievement, by reanalyzing PISA data. Specifically, we scrutinize the construct validity of the OTL measure in PISA that has been used in previous research. Our analyses reveal two latent dimensions of the OTL indicators in PISA, namely an unbiased OTL dimension and a self-concept dimension. The relationship between social background and mathematics achievement was only weakly mediated by OTL, when effect of students' self-concept was controlled for. Our results suggest that the previous research finding that schooling perpetuates social gaps in mathematics performance suffers from a construct validity problem.