Sorry for my multiple posts, but the issue of virus transmission is important.
Masks are the most important method of mitigating transmission. Some studies suggest they reduce it by 70%, more than hand washing and spatial separation. Obviously most cabs are fairly leaky and there is good air circulation. Equally the incidence of the virus needs to be considered, as well as the risk to the instructor and student in terms of age, ethnicity and comorbidty. However many professional pilots and cabin crew are successfully using and working with masks. The reusable masks are easy to use because they do not need professional fit testing. I wear mine for 6 hours at a time and I am happy to answer PMs about various models and how to don / doff / test. CO2 retention is a new one on me and I cannot understand the physiology. The dead space created by a mask is insignificant, rebreathing is insignificant. Altitude should not increase CO2........S-Works please do send me any papers you have on that as I am genuinely interested.
Each person has to be comfortable with the precautions on their flight and S-Works' second paragraph is spot on. However it is not true to say masks and gloves are pointless.