This paper aims at investigating the effect of Economic Social and Cultural Status (ESCS) on the educational performances of students, using two waves of the Programme for International Student Assessment (PISA) survey of OECD (2009, 2012). The analysis is conducted at student level for all countries included in the PISA sample. The estimates are based on the Conditional Data Envelopment Analysis (DEA), applied for the first time in the Slack Based Measure (SBM) version. This method allows a detailed evaluation of the additional effort the students should do when they are operating in an ESCS that has a comparative disadvantage. The unprecedented use of a conditional non-radial model of efficiency, provides evidences of a significant effect of the ESCS on performances, with a strong heterogeneity among students, and countries. It follows that some problems with education may not be due to the education systems themselves, but to the economic, social and cultural gaps, which determine a persistence of inequality of opportunity. Thus, public policies are needed to foster virtuous paths to reduce disparities among students with different socioeconomic background both between and within systems.