In the same report were noted other infections in the same school in different classrooms. One case was a cluster of three who had spent a night together at a sleepover (presumably unmasked and at close distance). No other students in that masked classroom were affected. Since the researchers were testing all the students to track the course of the infection, they found 4 other students in separate classrooms who tested positive, but none of their classmates did. That is strong evidence (not an anecdote) that masking students in school situations reduces the risk of transmission.
Another point I forgot to include -- the teacher was unvaccinated. If the school district cannot enforce a vaccination requirement for teachers, can they at least provide informed consent to parents, warning them which teachers are not vaccinated?