Before you get going about your favourite 'lunch hour', zoom into the attached figure (from doi.org/10.1016/j.jo...) notionally similar classrooms and occupancy - so ventilation behaviour has a leading order effect.

The article goes to great lengths to infer ventilation rates concluding the attached - these rates account for inoccupancy and are far more meaningful than CO2 thresholds. The fact these are low and adherence high is the best evidence for the need of changing things for the better...