They can do more. The HVAC systems should increase air exchange, to reduce recirculation, and likely need higher output (more cfm - larger ducts, more ducts, and/or higher velocity).
Very few schools in the UK have a HVAC system. Some of the more modern ones perhaps, but the majority of schools in the UK are likely older than the students grandparents.
True. I'm just saying that technically they could upgrade (or implement) the HVAC system to meet the new demands. Working with the existing hardware and not making any changes makes the CO2 meters practically worthless. Now we've added the stress or placebo effect of knowing that it's a detrimental environment on top of it actually being one.
If all they achieve is getting the windows opened that is already a huge success.
I would expect adding a simple fan to speed up the air exchange would help a lot too...
I would hope. Although based on other comments it sounds like even with the windows and doors open it's still high. Maybe a window fan would be beneficial, but I wonder if the heating system would keep up with it in the winter.