Although numerous studies document different forms of discrimination in the U.S. public education system, very few provide plausibly causal estimates. Thus, it is unclear to what extent public school principals discriminate against racial and ethnic minorities. Moreover, no studies test for heterogeneity in racial/ethnic discrimination by individual-level resource needs and school-level resource strain -- potentially important moderators in the education context. Using a correspondence audit, we examine bias against Black, Hispanic, and Chinese American families in interactions with 52,792 public K-12 principals in 33 states. Our research provides causal evidence that Hispanic and Chinese American families face significant discrimination in initial interactions with principals, regardless of individual-level resource needs. Black families, however, only face discrimination when they have high resource needs. Additionally, principals in schools with greater resource strain discriminate more against Chinese American families. This research uncovers complexities of racial/ethnic discrimination in the K-12 context because we examine multiple racial/ethnic groups and test for heterogeneity across individual- and school-level variables. These findings highlight the need for researchers conducting future correspondence audits to expand the scope of their research to provide a more comprehensive analysis of racial/ethnic discrimination in the U.S.