Marine pollution from antrhopogenic waste is a serious environmental issue, with negative impacts on ecosystems, species, society and economy. In particular, microplastics can enter food chains and pose a threat to both human and marine health. Environmental monitoring and citizen engagement (citizen science) are effective tools in addressing this issue. The “Guardiani della Costa” (Coast Guardians) program, promoted by the Costa Crociere Foundation, involves high school students in coastal monitoring. In spring 2023, 2,902 students from 102 classes surveyed natural and human-made debris, including micro- and meso-plastics, following rigorous protocols at 897 monitoring sites across 92 beaches. The results show that 21% of the debris was of human origin, with 47% of them being plastic. Micro and mesoplastics averaged 24.2 debris per m², with higher concentrations in the backshore. The findings demonstrate the potential of citizen science in environmental monitoring and for raising awareness among young people about coastal conservation.