Food Waste is a global significant issue for ethical, environmental and economic reasons, while its management is difficult due to its frequent low visibility. Individual choices and preferences are closely related to the generation of food waste although likely to be modified through education and awareness campaigns. In particular, school canteens are big generators of food waste and, at the same time, provide a great opportunity to improve habits regarding nutrition and education on sustainability, thus impacting the future of the food system. The end purpose of this research is identifying the causes of food waste and unveiling best practices towards its reduction. To achieve this goal, we have designed and developed a mixed methods research approach including semi-structured interviews with managers and staff in schools and catering firms and waste audits at four school canteens - measuring waste from over 10,000 pupil's trays. In order to avoid potential bias due to meal preference, the audit lasted three to five consecutive weekdays per school, thus comprising different menus. We estimated overall food waste between 60 and 100 g per pupil per day. Plate waste represented the highest source of waste, although a big disparity was found among the schools based on their different educational perspectives. Key food waste determinants found were: first, top management standpoint towards food waste and sustainability in general. Secondly, we observed relevant differences among the three catering business models studied, regarding the stages where food waste is usually produced. Finally, food waste was also impacted by the diverse resource availability among the schools. Despite this, the human factor arose as the most relevant one when aiming to minimise food waste.