Poor ventilation in classrooms can increase the risk of infectious disease transmission, such as COVID-19, because it allows respiratory aerosol particles that may contain viruses to accumulate. Air purifiers can effectively reduce transmission rates in community spaces, including classrooms, because they increase the air change rate in the room and reduce particle concentrations. In this study, we investigate the effectiveness of Corsi-Rosenthal Boxes (C-R Box) in reducing particle concentrations in active, occupied classroom settings. A C-R Box is a do-it-yourself, cost-effective alternative to commercial air purifiers built from a box fan, four readily available filters, cardboard, and duct tape. We collected measurements of coarse (particles with diameters > 2.5μm) and fine (particles with diameters 0.5μm - 2.5 μm) particle number concentrations and PM2.5 (particles with diameter < 2.5μm) mass concentrations. Specifically, we compared measurements in occupied classrooms before and after we turned the C-R Boxes on. In our testing, C-R Boxes reduced fine particle number concentrations by 56-91% and PM2.5 mass concentrations by over 70% after we turn on the C-R Boxes. We also simulated velocity profiles in the classrooms with running C-R Boxes showing mixing throughout the classroom ensuring that all air can encounter the filter.