Inequality of opportunity strikes when two children with the same academic performance are sent to different quality schools because their parents differ in socio-economic status. Based on a novel dataset for Germany, we demonstrate that children are significantly less likely to enter the academic track if they come from low socio-economic status (SES) families, even after conditioning on prior measures of school performance. We then provide causal evidence that a low-intensity mentoring program can improve long-run education outcomes of low SES children and reduce inequality of opportunity. Low SES children, who were randomly assigned to a mentor for one year are 20 percent more likely to enter a high track program. The mentoring relationship affects both parents and children and has positive long-term implications for children's educational trajectories.