Yes, it is indeed amazing how fast CO2 levels can increase. This is especially a problem in low-ventilated classrooms.

At AirGradient we measure a lot of classrooms and it is not uncommon to see CO2 levels rising to above 4500ppm just within 2 or 3 hours. We wrote a blog post some time ago highlighting this [1].

[1] https://www.airgradient.com/open-airgradient/blog/we-measure...