Although prior research has examined children's perceptions of the classroom environment as related to academic achievement, genetically sensitive designs have not been employed. In the first study of its kind for the primary school classroom environment, data were collected for 3,020 pairs of nine-year-old identical and fraternal twin pairs in same and different classrooms on their perceptions in six domains: social integration, opportunity, adventure, general satisfaction, negative affect, and teachers. Data were also collected for teacher-assessed academic achievement (ACH). Modest genetic influence was found for children's perceptions of the classroom environment: an average of 0.33, 0.06, 0.25, 0.27, 0.19, and 0.20 of the variance, respectively. Non-shared environment played a more influential role, accounting for an average of 0.58, 0.78, 0.64, 0.60, 0.69, and 0.65 of the variance, respectively. Negative affect, adventure, social integration, and opportunity were significantly, albeit modestly, associated with ACH. Results suggest that perceptions of the classroom environment are driven primarily by child-specific experiences, and that such perceptions, although experientially important, are less important for ACH. (Contains 2 figures and 4 tables.)