Well, except for that time toward the end of last year when the heating broke.
Anyway, so yes, it gets very hot. Fortunately, the teachers are well aware of our plight as students, mainly because they're also in the heated room with us so they're sort of going through the same thing, and they take measures to make sure we don't a) collapse, or b) die. These measures basically involve opening a window or a door, or a combination thereof.
Of course, they don't do this too frequently because then all the hot air would just float away and you'd just be in a normal room, which kind of defeats the object. And by 'kind of' I mean 'completely.'
As a general rule of thumb, then, I go on the expectation that we'll sizzle for the standing series of postures, then get a window treat around the midway point, thereafter followed by an occasional treat every now and then until the 90 minutes is up.
Believe me, a window being opened has never been so exciting. If you're flagging a bit, it's basically like all your Christmases have come at once. And the studio where I practice really does get a nice through-flow of air if both the windows and the door are opened. You'd never believe how rejuvenating a blast of cool air could be.
Not everyone likes a nice breeze, though, so more often than not they don't stay open very long. As a result, I have come to the conclusion that I have two means at my disposal to keep cool if the teacher's being particularly stingy. The first is simply to tip water over my head, which is both quick, easy, and effective; the second, which involves an attempt at bribing my teachers before class, has yet to prove successful - although I did buy some additional open-window time once with an angrily-raised eyebrow.