There are few things more stultifying than a closed-up, warmed-over classroom. The stuffy class is not only uncomfortable to be in, it also undercuts your noble efforts to fill each student's cranium with knowledge. The mind wanders, eyes glaze over, and learning comes... to... a... complete... standstill.
So open the windows. Fresh air can perk up the class and have positive effects on mental development and attitude. When the weather is nice, a bit of a breeze has other benefits. Researchers at the University of Michigan, for instance, found that even a small dose of warm weather can help improve mood and memory, while increasing our willingness to accept new information.
Break out of your temperature-controlled cocoon. Indulge your oxygen-starved soul. Breathe deep. Repeat.