The standard classroom can create risk for increased COVID exposure. It would be difficult to define a standard classroom since every school can be different in terms of HVAC. So we will explore this, just a little.
HVAC for public buildings has had a design purpose of saving energy. It is likely to also have had some sense of occupancy load by measuring internal humidity. This occupancy load would have been coupled to fresh air intake system that would have original design intent to prevent over-accumulation of CO2 in the building. If the air is mostly recirculated, the exhaled COVID particles will grow in concentration over time. The resulting COVID concentrations could make exposure more and more likely. Therefore we are now in need of venting COVID accumulation outside.
A simple solution in good weather, is to hold classes outdoors. Even under a roof, the side breezes would mix a great deal of fresh air into the classroom air, preventing the concentration from being particularly potent. Temperature would be controlled by the weather, so when the weather is pleasant, the classroom would be, too.
When following appropriate guidelines, and with adequate equipment, increasing ventilation can be done in a closed classroom to reduce COVID concentrations. In some buildings it is an adjustment. In others new equipment must be added. If it is cold out, there will be an additional need to warm the room because of the air exchange, but some of that energy can be saved by transferring heat from the outgoing air to the incoming air with a heat exchanger. By having each room's ventilation independent, only occupied rooms would need to use additional heat energy if sensing is designed to handle that.
The purpose of laying out this sketch is not to be complete in informing the reader at all. The point is mostly to introduce the reader to the point that this is an area of some complexity that is ripe for analysis and expert-assisted solutions.
We at Engineers for a Sustainable Future have discovered information that we believe should be of interest to people who need to decide how to properly use their buildings in this environment when COVID has suddenly changed our rules.
ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) is a society of professional engineers that are expert in heating and air conditioning, one set of key topics necessary to understanding handling COVID in buildings.
They have designated a certification “CHD” - Certified HVAC Designer which looks like it may be the kind of expert you would like to use to determine what your building would need for a certain pattern of use. Their website talks about this HERE. and HERE. and their home website HERE.
They have obviously taken this topic seriously and have generated a great volume of information to help. I would suggest that even an intelligent building operator should not quickly consider himself/herself an expert just because of reading their material. Actually contacting one of their experts is probably the step to real assurance.
As we all know, COVID is only partially understood. What kills the virus and what contains it is not fully clear.