This study examined the role of observed classroom quality in children's task-avoidant behavior and math skills in kindergarten. To investigate this, 1268 children were tested twice on their math skills during their kindergarten year. Kindergarten teachers (N = 137) filled in questionnaires measuring their professional experience and also rated the children on their task-avoidant versus task-focused behaviors. Trained observers used the CLASS instrument (Pianta, La Paro, & Hamre, 2008) to observe 49 kindergarten teachers (out of 137) on their emotional support, classroom organization, and instructional support. The results of multilevel modeling showed that kindergarten classrooms differed in terms of children's task-avoidant behavior and math skills. Moreover, the more instructional support was evidenced in the classroom, the less children were rated as showing task-avoidant behavior. Task-avoidant behavior then predicted children's low levels of math skills. The findings of the present study emphasize the importance of quality of instructional support for children's adaptive classroom behaviors.