The goal of this study was to investigate the transactional link between the affective quality of teacher-student relations and students' externalizing behavior in upper elementary education. We studied teacher support and conflict separately and examined whether associations differed for boys and girls. Data were collected from 1452 Dutch fifth graders (Mage = 10.60 years) at three time points within one school year, including peer nominations of teacher-student relationships and external observations of teacher-student interactions. We used random-intercept cross-lagged panel models to examine the associations within the school year. Student behavior and teacher conflict and support were clearly interrelated within measurement moments. That is, within each time point, deviations from students' typical level of externalizing behavior were associated with deviations in teacher conflict and support in teacher-student relations. In contrast to earlier work, we found no transactional link between students' externalizing behavior and their relationships and interactions with their teacher over time, neither for teacher conflict nor for support. However, for boys, an association was found between externalizing behavior and later increased teacher conflict. We concluded that it remains important to invest in supportive teacher-student relations to prevent increasing conflict and that transactionality may occur within shorter time intervals.