Plastic debris in aquatic and marine environments often breaks up into fragments that are smaller than 5 millimeters, which are then classified as microplastics. While there is not yet a standardized and validated methodology for characterizing microplastics, the protocol developed in this study uses methods for isolating and observing microplastics and for the investigation of how they interact with organisms present in biofilms from urban waterways. Project-based learning (PBL) has been proven to be a successful strategy in K–12 science education; the implementation of PBL provides opportunities for student-driven inquiry and provides teachers with a means to integrate curriculum with current research and to consider the effects of human impacts on the environment. This paper describes the protocol developed for high school teachers to educate students about microplastics and how to successfully isolate and observe them. Teachers and students in Maryland successfully isolated microplastics from biofilm samples from the Inner Harbor, Baltimore, Maryland, and shared their results. International teachers and students in Barcelona, Spain, involved in a related project, had similar results and shared experiences through images, video, and online meetings. These collaborations provide important opportunities for student-driven inquiry and for them to engage in methods of current scientific research.