Background: The role of reactive aggression in the development of peer victimization remains unclear due in part to a failure to account for confounding problems of behavioural undercontrol (e.g., hyperactivity). As well, the school social context has rarely been examined to see whether these risks are mediated by relationships with teachers. Aims: This study tests the prospective relations between reactive aggression, hyperactivity, victimization, and teacher-child (T-C) relationship, to determine whether conflict mediates the relationships between externalizing problems and victimization. Sample: A sample of 1,114 Australian students were followed from pre-kindergarten through first grade. Methods: Cross-lagged path analyses were conducted, with comparison of gender-moderating models and autocorrelation models. Full-information maximum likelihood was deployed to account for missingness. Results: Best fitting models found that the relationship of early externalizing problems to later victimization was mediated by T-C conflict. No evidence of victimization increasing externalizing problems nor gender differences were observed. T-C conflict in kindergarten predicted subsequent increases in victimization, reactive aggression, and hyperactivity. Conclusions: Understanding the processes whereby externalizing problems confer risk of victimization involves understanding the whole social context of classrooms, including relationships with teachers. Finer-grained research is needed to better understand how peer dynamics may be influenced by observation of T-C relationships. Pre-service teacher education needs to ensure a focus on the potential social impact of teacher's relationships with students.