I teach at a secondary school in the UK, in a classroom with no external windows (but with air quality monitors). After 1 hour of 30 teenagers the co2 will be at around 2000-2500ppm which I can confirm is stuffy. Highest I've seen is in the next door classroom which made it up to 3800ppm back in the summer.

It really does make you (and the kids) feel really dopey, so not exactly ideal.