The present study examined reciprocal influences between student-teacher relationship quality and students' externalizing and internalizing behaviors in secondary education. Our sample included 1,219 secondary school students (49.1% boys; M[subscript age] = 13.53, SD = 1.77) from seventh, eighth, tenth, and eleventh grade. Students reported about their externalizing and internalizing behaviors and the quality of the relationships (closeness, conflict) with their Dutch and math teachers at three occasions within one school year. Cross-lagged path models showed positive reciprocal associations between conflict with the math teacher and externalizing behavior over time. For the Dutch teacher, externalizing behavior positively predicted conflict over time, but not the other way around. Externalizing behavior also negatively predicted closeness with both teachers over time.