it varies, but usually opening the windows is plenty of ventilation unless it's not an option (e.g., because it's too hot or too cold outside)

it's much more common to wish for bigger windows because you're hot than because you're falling asleep

(1000 ppmv - 420 ppmv) co₂ = 580 ppmv co₂

r 20° / 1 atm = 24 ℓ/mol

1 kg co₂ / day / 人 = 11.6 mg co₂ / s / 人

30 人 · 11.6 mg co₂ / s / 人 / (44 g co₂/mol co₂) = 7.9 mmol co₂ / s (a classroom)

7.9 mmol co₂ / s · 24 ℓ/mol = 0.19 ℓ co₂ / s

0.19 ℓ co₂ / s / 580 ppmv co₂ = 330 ℓ / s (of air)

in medieval units that's about 700 'cfm'

if the classroom has 10 m² of open windows you're fine, even an undetectable draft of 0.03 m/s is sufficient, but if it has 1 m² you need a barely detectable breeze of 0.3 m/s, if it has 0.1 m² you need a pretty noticeable breeze of 3 m/s ('light breeze' on the beaufort scale), and if it has a little 100-mm-square register you need 30 m/s which is a 'violent storm' on the beaufort scale (above 'whole gale')

at 'fresh breeze' (8–11 m/s) you need 300–400 cm² of window

well designed ventilation systems can provide this kind of airflow while controlling the temperature and humidity but they are at a big disadvantage because 300–400 cm² of window is a lot smaller than 300–400 cm² of duct

sometimes that's not what matters most tho