In fact, designing a high-filtration but transparent mask or face shield might be an important solution in classrooms as well, to help keep teachers safe.
When windows cannot be opened, classrooms could run portable HEPA filters, which are capable of trapping viruses this small, and which sell for as little as a few hundred dollars. Marr advises schools to measure airflow rates in each classroom, upgrade filters in the HVAC system to MERV 13 or higher (these are air filter grades), and aspire to meet or exceed ASHRAE (the professional society that provides HVAC guidance and standards) standards.
This is a really interesting article. I am not sure how long I could keep windows open in my classroom as we move through autumn into winter.