The objective of this work was to engage students from public schools of São Paulo coast (Brazil) in the study and control of litter on beaches. Two public schools from coastal cities of São Paulo State (Peruíbe and Ubatuba) had students trained to sample and quantify marine litter on beaches. After this training, using Google Earth images, two beaches of each city were chosen to be studied. On each beach, samples were taken in the 100 meters of the selected stretch. The residues were classified by type of material and type of item (seeking to associate to sources). After the sampling surveys, the students were oriented to the data analysis. The students prepared a presentation with sampling results, hypotheses to explain the possible origin of the marine litter and measures that could be taken to reduce the problem on the studied beach. For both schools, this workshop revealed great students’ involvement during all process, from the field-work and data analysis, to the development of hypothesis and solution propositions. The beaches of Peruíbe were more isolated and in one of them most of the waste was coming from the sea, which carried litter from boats and nearby villages. Meanwhile, the beaches of Ubatuba were urbaner and had high occurrence of construction materials, indicating a miss-managed discard of litter by local residents. The main outputs of these workshops were: (1) giving the students an experience of how a scientific project is developed, (2) generating reliable data on marine litter in the school areas, (3) stimulating discussion about sources and proposing solutions and (4) establishing a cooperation network between the school and the university.Keywords: marine litter, monitoring, beaches, citizen science