We have air conditioning in about 50% of the middle school, about 25% of N*, and very little at West. GOAL has no air conditioning. A few years ago, I asked an architect what he thought it would cost to provide air conditioning throughout the elementary buildings. He estimated $300,000 - $500,000 per building, but couldn't give an accurate estimate unless a thorough review of buildings was done.
The decision to dismiss early isn't based only on outdoor temperatures. The humidity is a factor, as is overnight temperatures. If overnight temperatures stay near 75', the buildings don't cool off in time for the next school day and rooms stay hot. The length of the "heat wave" is also a factor. Most times, one hot day does not result in an early dismissal. When we have 7-10 days of excessive heat in a row, buildings stay warmer.
We try to take precautions - we open doors and windows early in the morning and run fans in the halls to help circulate the morning air. Students have water bottles at their desks and are encouraged to take water breaks. We reduce or eliminate afternoon recesses to prevent students from overdoing it. Outdoor practices are held in the morning or later in the evening.