This study examines the effects of family socio-economic disadvantage and differences in school resources on student achievement in the city of Cartagena, Colombia. Using data from the ICFES and C-600 national databases, we conduct a multilevel analysis to determine the unique contribution of school-level factors above and beyond family background. The results from the hierarchical linear models show that while family socio-economic background significantly affects student achievement, school composition and school resources explain as much as half of the effects of family background. More specifically, the achievement gap in public schools is explained in large part by differential resource allocation and concentration of poor students in public schools, which in turn lowers student achievement. (Contains 1 figure, 4 tables and 1 note.)