The aim of this study was to assess whether children's development benefited from teacher-and child-managed academic activities in the preschool and kindergarten classroom. Extensive systematic observations during four half-days in preschool ("n"?=?8) and kindergarten ("n"?=?8) classrooms revealed that classrooms differed in the amount of time spent on teacher-managed academic activities. The time teachers spent on math activities was remarkably low. Teacher-managed activities accelerated children's development, particularly for preschool children ("n"?=?47); they showed larger gains in language, literacy, and math skills if their teacher devoted a relatively large proportion of classroom time to these topics. With regard to kindergarten children ("n"?=?45), only their math skills seemed influenced by their teacher's engagement in academic activities; these children showed larger gains in math skills if their teacher initiated many language and literacy activities. Children's language and literacy development benefited from child-managed language and literacy activities. In contrast, child-managed math activities were not related to children's gains in math skills. The study provides insight into children's daily experiences in the preschool classroom and yields important implications for the professional development of teachers in early childhood education.