Out-of-school influences such as families and neighborhoods account for the vast majority of differences in educational achievement among students in the U.S. We report the results of an intervention aimed at increasing the effort and effectiveness of parents in supporting their children’s success in school. We find that weekly teacher-to-parent one-way communication in the form of messages from teachers sent to parents increased the probability a student earned course credit during a summer credit recovery program by 6.5 percentage points, a 41% reduction in the number of courses failed. Much of this effect is driven by parents who were randomly assigned to receive messages that focused on what their students needed to improve in class (as opposed to messages that focused on what students were doing well in class). Increases in passing rates can be attributed almost exclusively to preventing students from dropping out of the summer program, rather than by reducing failure or dismissal rates. A cost-benefit analysis suggests that teacher-to-parent communication programs compare very favorably to other educational interventions. These results suggest that education policymakers may be underinvesting in strategies to leverage one of the largest influences on students’ academic success: their parents.