Objectives: To explore whether an intervention during mandatory schooling can lead to age-specific changes in water safety knowledge and attitudes. Methods: Age-specific questionnaires were distributed to 202 kindergarten and grade one pupils, 220 elementary school pupils and 337 pupils attending the first three high school grades in Greater Athens. The information was used to design an educational package that was subsequently presented to pupils of the same grades and similar socio-demographic profiles attending different schools in the same area. One month later, a post-exposure evaluation was conducted using the initial questionnaires, in which 115, 205 and 321 pupils from the respective grade categories provided their responses. In order to compare the performance of pupils exposed to the educational intervention with that of pupils who participated only in the initial assessment, mean differences in scores measuring overall knowledge and attitudes were estimated within each of the three grade groups adjusting for age, gender, sibship size, maternal education and swimming knowledge. Results: Among kindergarten and grade one pupils, those who received the intervention scored significantly higher for knowledge (17.40%, 95% CI 6.41% to 28.39%) and attitudes (23.64%, 95% CI 4.48% to 42.79%). Among elementary school pupils the gains in knowledge were less evident (14.58%, 95% CI -3.05% to 32.21%)) and almost null in attitudes (5.64%, 95% CI -11.47% to 22.77%). Further advancement of age showed no improvement in knowledge (-0.15%, 95% CI -5.30% to 4.99%) and a minimal, insignificant increase in attitudes (6.32%, 95% CI -1.87% to 14.52%) among exposed high school pupils. Conclusion: The school-based intervention resulted in considerable positive changes in knowledge and attitudes among very young pupils. Elementary schooling seems to provide meagre opportunities to simply improve knowledge. Alternative/complementary approaches should be sought in any attempt to modify behavior.