“Exposure to CO2 can result in drowsiness, eye irritation, and inability to concentrate. If a ventilation system provides inadequate fresh-air intake, levels of CO2 in the classroom can rise dramatically during the course of a classroom session.” Brian W. Smith
Have you ever tried reading a book while feeling drowsy? It’s not unusual to read sentences several times before they really sink in which is not a very efficient way to learn. It’s even worse being drowsy while taking a test. It’s difficult to get high marks when concentration is lacking.