Eco-schools aim to improve the environment through direct and indirect effects. Direct effects are those that result from the implementation of an environmental management system. Indirect effects are educational gains. The current study examines the effectiveness of eco-schools concerning three student outcomes: (1) environmental knowledge, (2) environmental attitudes, and (3) environmental affect. The study includes 1287, 10-12-year-olds from 59 schools (38 eco-schools and 21 control schools). Multivariate multilevel regression analyses show that eco-schools mainly influence their students' environmental knowledge; they do not influence environmental affect. Eco-school students, furthermore, have equal preservation attitudes and lower utilization attitudes, as compared to control-school students. The implications of these results for research and practice are discussed. (Contains 3 tables and 1 figure.)