Extensive research on the effects of education programs has left a key question unanswered: how many students actually benefit from these programs? We answer this question by re-analyzing the microdata from the universe of published education RCTs from developing countries. Despite the current enthusiasm for replication, we were able to obtain data for just 42% of the studies in our sampling frame. Our analytic sample includes more than half a million observations covering 119 different interventions run in 24 countries. Using this data, we construct non-parametric lower bounds on the across-student variance of treatment effects for each intervention. Our meta-analytic estimate of the standard deviation of treatment effects is 0.08 SDs of control-group test scores---over 30% larger than the average treatment effect for the studies in our sample. Moreover, the standard deviation of impacts varies widely, from nearly zero for 20% of programs to almost half an SD of test scores for the top handful of interventions. The average intervention leaves at least 1 in 20 students behind, with some programs having lower-bound rates of nearly a third. The variance of treatment effects is strongly correlated with a range of study-level factors. It is systematically larger for programs with larger treatment effects, and for pedagogy-focused interventions and programs targeted at entire schools. It is smaller at the preschool and secondary school levels compared to primary-school programs, and is larger for government-run interventions as compared to those run by NGOs and private companies. However, we can explain almost none of the variance in treatment effects using \textit{individual}-levelcovariates from the studies, even if we use machine learning methods to estimate and partial out CATEs. In other words, some interventions have more variable effects, but we cannot identify who benefits from them and who is harmed. Our results suggest that education interventions in developing countries are leaving a large number of children behind.