At best it is warm for large parts of the day, at its worst it is very hot here right now in Korea. Most buildings you go into all have their air conditioners on or at the very least have some fans. My workplace has good air conditioners. There are three large ones for the third floor for reception, the computer study room and the teachers room.
In the classrooms their are two vents where the air is filtered in and keep the room nice and cool. You only really notice the heat when you are on the stairs or if some sneaky teacher turns the air con in the teachers room when they have a break.
It is very important that the air con is a) on, and b) works well. Otherwise it would be unreasonable to expect classes to go well. You can't teach or study without it really. Before the full breakout of Summer the classrooms were getting toasty. In May the kids were screaming for the air con to go on. And on it went, probably will be till late September. But then the kids started complaining. "It's too cold", they screamed. Again and again. And now, one third of the way through July I am getting requests for the air conditioning to be turned off. No, it's not going to happen, not in my classroom.
Having dated and subsequently married a Korean and also having worked with people here for over four and a half years I believe that Koreans often struggle with temperature changes. I don't know why. It gets hot here every Summer and it gets cold ever Winter but I cannot begin to say how many times I've heard "It's hot" or "It's cold" in the office when it has been anything but. The air con stays on, unless a Korean co-worker decides they don't want it on in the teachers room it would seem.