This article studies the effects of the neighborhood in which a school is located on children's mathematics achievement in Chile. It uses data taken from a sample of 127,020 sixth grade students measured by the National Education Quality Measurement System [Sistema Nacional de Medición de la Calidad de la Educación]. The incorporation of a measurement of socio-economic polarization of the geographic environment, which is innovative in urban studies, allows us to qualify some critical aspects suggested in the academic discussion. A lagged dependent variable model is used, controlling for the score obtained by the same students in fourth grade. Using multilevel linear regressions, the results show positive effects related to participation in neighborhood organizations. One critical finding is that socio-economic polarization has a negative and significant impact on the educational achievement of sixth graders. The conclusions highlight the repercussions associated with acute inequalities in the neighborhoods, and speak to the importance of accessing dimensions which are more closely linked to cities' social structure.