“In the current study, most COVID-19 cases among students and staff were acquired from the community and approximately 10% of cases were acquired within school. The researchers found that for every 100 community-acquired cases, school districts with mandatory masking had approximately 7.3 cases of in-school infections, while optionally masked districts had 26.4 cases of in-school infections. In other words, school districts with optional masking had approximately 3.6 times the rate of in-school COVID-19 cases when compared to schools with mandatory masking. These data also show that mandatory masking was associated with a 72% reduction of in-school COVID-19 cases, compared to districts with optional masking.”

From Pediatrics:

https://publications.aap.org/pediatrics/article/149/6/e20220...