Early school leaving (ESL) is considered one of the most serious problems in the functioning of European education systems. In particular, Spain has traditionally exhibited one of the largest ESL rates in Europe. This study aimed to shed light on the role of the month of birth as a factor conditioning the probability of ESL. Using high-quality census data and a regression discontinuity design, we documented that Spanish males from lower social strata born before the school-entry cut-off are substantially more likely to be observed as early school leavers than their counterparts born after that date. We do not find such effect among females or males from other social strata. Overall, educational policy should target more seriously the inequalities associated with the month of birth as they do not vanish before affecting relevant decisions such as leaving education early, at least for males from lower social strata.