Since education aims to offer applicable knowledge, studying knowledge--behavior relationship is of key importance in promoting environmental education. But there is scarcity of studies addressing the relationship between knowledge and real behavior choices. We examined the connection between environmental knowledge and behavior (self-reported and measured in an operative way) among 10-12-year-old students, with special focus on the potential mediating role of attitudes. The main research question was whether knowledge (measured with an achievement test) correlates with actual environmental behavior, and how mental accessibility of environmentalism is related to the relationship between knowledge and actual behavior (i.e. choosing a material for completing a manual task). The study with 325 persons revealed that although the positive connection between knowledge and self-reported behavior was fully mediated by environmental attitudes, knowledge was just slightly related to actual behavior, even when the topic of environmentalism appeared before the behavior choice. However, behavior was related to school, suggesting that school-level socialization (beyond the knowledge transfer) is highly influential in forming environmental behavior. The difficulties of studying actual behavior and implications of our findings for practitioners from the field of environmental education are discussed.