This longitudinal study investigated the associations among children’s externalizing problems, task-avoidant behavior, and academic performance in early school years. The participants were 586 children (43% girls, 57% boys). Data pertaining to externalizing problems (teacher ratings) and task-avoidant behaviors (mother and teacher ratings) were gathered, and the children were tested yearly on their academic performance in Grades 1-4. The results were similar for both genders. The analyses supported a mediation model: high externalizing problems in Grades 1 and 2 were linked with low academic performance in Grades 3 and 4 through increases in task-avoidant behavior in Grades 2 and 3. The results also provided evidence for a reversed mediator model: low academic performance in Grades 1 and 2 was associated with high externalizing problems in Grades 3 and 4 via high task avoidance in Grades 2 and 3. These findings emphasize the need to examine externalizing problems, task-avoidant behavior, and academic performance conjointly to understand their developmental dynamics in early school years.