Background/Context: While many studies show that greater economic inequality widens the achievement gap between rich and poor students, recent studies indicate that countries with greater economic inequality have lower overall student achievement. Purpose/Objective/Research Question/Focus of Study: This study explores whether family inequalities (family income) or school inequalities (educational materials or teachers with university degrees) reduce overall student achievement through micro-economic mechanisms, such as fewer educational resources (via rent-seeking) or inefficient resource allocation (via diminishing marginal returns). Population/Participants/Subjects: The Organization for Economic Cooperation and Development's Program for International Student Assessment (OECD-PISA) selected 475,760 representative fifteen-year-olds and their principals from 18,094 schools in 65 countries. Research Design: In this secondary analysis, we tested whether family or school inequalities were related to students' mathematics test scores, and whether fewer educational resources or inefficient resources allocation mediated these relationships. Data Collection and Analysis: Each student received a mathematics test. The students and their principals also received a questionnaire. World Bank economic data on each countries were merged with the OECD-PISA data. To analyze this data, we used item response models, Warm indices and multilevel analyses. Findings/Results: In countries with greater family inequality (GDP Gini) or school inequalities (of educational materials or teacher quality), students had lower mathematics achievement. The results were similar in all student subsamples (high vs. low SES; high vs. low achievement). As the mediation results for each inequality differed, they suggest that these inequalities operate through different mechanisms. Family inequality and school inequality of teacher quality are linked to fewer teachers with post-secondary education and lower mathematics achievement. Meanwhile, school inequality of educational resources is linked to diminishing marginal returns and lower mathematics achievement. Conclusions/Recommendations: Family inequality and school inequalities (educational materials, teacher quality) are distinct inequalities that are all linked to lower mathematics achievement, but not substantially correlated with one another. Thus, each inequality can be addressed separately. As none of the subgroups of students (not even the richest ones) benefit from any of the inequalities, disseminating the results widely can help more laypeople (especially the richest ones) recognize their mutual benefit in reducing these inequalities--or reduce their inclination to support policies that exacerbate these inequalities. As reducing family inequality can be extremely costly and politically controversial, a strategic intervention at the inequality mechanism level (e.g., increasing teacher quality in schools with few high quality teachers) might be improve mathematics achievement more effectively.