Previous research has provided evidence for developmental cascades between externalizing and internalizing problems via mechanisms such as peer and academic problems; however, there remains a need to illuminate other key mediating processes that could serve as intervention targets. This study, thus, evaluated whether developmental associations between aggression and internalizing are mediated by teacher—as well as peer—relationships. Using data from z-proso, a longitudinal study of Swiss youth (n = 1523; 785 males), an autoregressive latent trajectory model with structured residuals (ALT-SR) was fit over ages 11, 13, and 15 to examine within-person developmental links between aggression, internalizing problems, and the mediating role of peer and teacher relationships, while disaggregating between- and within-person effects. Teacher and peer relationships did not play a role in the progression of externalizing to internalizing problems or vice versa, however, teacher and peer relationships showed a protective effect against developing internalizing problems at ages 13. The results suggest that good quality relationships with teachers in early adolescence can help prevent internalizing problems from developing.