Operable Windows Needed in Schools for Natural Ventilation Amid Pandemics

The CDC on school ventilation in regards to Covid-19 reads: https://www.cdc.gov/coronavirus/2019-ncov/community/schools-childcare/schools.html

• Ventilation
  • Ensure ventilation systems operate properly and increase circulation of outdoor air as much as possible, for example by opening windows and doors. Do not open windows and doors if doing so poses a safety or health risk (e.g., risk of falling, triggering asthma symptoms) to children using the facility.

But what if most windows do not open or if a room has NO windows? Adding windows and retrofitting seems to be the logical solution or participate in distance learning.

Here is a website on natural ventilation from

https://www.commercialwindows.org/ventilation.php