Food waste (FW) has recently attracted the interest of different institutions and has been the focus of many studies due to its important environmental, social and economic impact. This paper aims to analyze whether a didactic intervention, consisting of informing teachers and pupils and involving pupils in reducing FW, could bring about changes in the level of knowledge and attitude towards FW and in the amount of FW generated during the mid-morning break and lunch at schools. This study was conducted at a public Primary School in Valencia (Spain). Subtle changes in the level of knowledge and attitude towards FW were detected in teachers and pupils after the intervention. Around 30% of FW reduction at lunch was observed in the intervention group but not in the other groups. A decrease of almost half of the average weight was observed during the mid-morning break in the rest of primary groups. The results apparently show that addressing the FW issue in classrooms can have a very positive effect on children’s attitudes. As it is such a cross-cutting issue; it raises awareness about a large group of Sustainable Development Goals, and encourages these young citizens to make conscious decisions and to act responsibly.