Because an awful lot of them don't have it.
Since late August, schools around Baltimore, Chicago, Cincinnati, Minneapolis and Fargo, North Dakota (Fargo, North Dakota!) have had to shut their doors and send their students home because of excessive heat. The start of the school year has, unfortunately, coincided with an end-of-summer heat wave across the Midwest that's now settling over the East Coast. As a result, the kind of northern school districts that regularly have to plan for "snow days" have been confounded instead by a different kind of climactic problem: They don't have air conditioning.
If you live in, say, Atlanta, Houston or Miami, this probably sounds unthinkable to you (I thought the same thing until I looked up my own 120-year-old elementary school in Chicago and realized it was never air conditioned, either). Newer and southern schools tend be air conditioned by default. But many northern school buildings predate the widespread adoption of air conditioning, and it won't necessary work to just stick window units in every classroom because of complications with air quality.
Now a troubling trend is coming into view: Cities that weren't built around the air conditioner are now starting to experience longer summers and hotter record-high days. Suddenly, the more mundane face of climate change looks like children who can't learn.
Outside of Boston, dormitory architects at Harvard have actually begun to prepare for such a scenario, building in room for infrastructure that could accommodate air conditioning in a future when it's really needed. Local school districts may want to think about it this way when weighing the investment of costly retrofits: Heat days will only get worse from here.
Top image: Flickr user Taber Andrew Bain.