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A large proportion of school meals are wasted, leading to missed opportunities to nourish pupils, environmental impacts, and economic losses. This intervention study evaluated the long-term efficacy of three educational approaches (giving feedback to guests via plate waste tracker, pedagogic meals, and kitchen workshops) in reducing plate waste in school canteens across Europe (Austria, Germany, and Sweden). Following the intervention, a sustainability assessment was conducted, including environmental, economic, and social perspectives. The results showed that the plate waste tracker significantly reduced plate waste, by 17% (4 g/guest) from an already lower baseline level of 23 g/guest, while demonstrating long-term efficacy with sustained waste reduction up to 15 months post-implementation. This reduction lowered the environmental impacts (by 212 kg carbon dioxide equivalents per school & year) and nutrient losses (1018 MJ, 12 kg protein, and 4 kg fiber per school & year), while proving cost-effective with a payback period of only 1–2 years. Therefore, despite upfront costs and implementation barriers, food waste reduction measures in school canteens provide substantial long-term benefits across environmental, economic, and social dimensions, making them a valuable investment for sustainable school meal programs.