This study deepens the relationship between School Library and Student Achievement taking into account an important predictor of achievement: the socio-economic and cultural backgrounds. The ESCS (Economic, Social and Cultural Status) Index is calculated on the basis of specific variables of the INVALSI Student Questionnaire. The data on School Libraries come from the School Questionnaire compiled by the School Principals in the course of the Self Evaluation Report (RAV); students’ achievement data come from INVALSI test results. We explore the role of School Libraries in schools with different ESCS. The schools examined are 7326, of which 5049 of Grade 5 and 2277 of Grade 10. The analyses show higher students’ performance in the schools with well-equipped and well-functioning School Libraries, the differences are greater in schools with Low and Medium-Low ESCS. A particular relevance seems to assume the School Library in Grade 5 in schools with low ESCS. These results are found for both disciplines investigated: Italian and Mathematics.