Syd Field (or maybe it was Robert McGee - one of those screenwriting gurus) liked to say that a movie was a magical universe - that is, that the movie is its own world, and that if its internal rules were sufficiently consistent and well constructed, you could forget about the real world while you were in its grip.
One of the organizing principles of the magical universe of Whiplash doesn't even bear a moment's scrutiny: the movie is set in a music conservatory in which, somehow, not a single student is female.
If you ask yourself why, a second aspect of the magical universe comes into sharp focus: If there were women in Fletcher's class, his drill-sergeant level of abusiveness would instantly make you wonder why nobody's sued this conservatory into the ground yet. As creepy as Fletcher's attacks on his students get - and they get very creepy indeed - if he were doing what he does to a woman, there would be an unavoidable current of sexual violence that would pull the entire movie in a very different direction than the one it's designed to head in.
So the movie makes an assumption of the sort that doctors have been making for generations: to keep things simple, let's just assume that all people are men.
Interestingly, it doesn't assume that they're all white men. It's jazz, after all: it would be really disconcerting if everyone in a New York City jazz band were white. But we never see Fletcher abusing a black musician, and I think it's for the same reason there aren't any women in the room: Fletcher's a terrible person who does terrible things, but the people he beats up on are privileged. (We only find out that one of them is gay late in the movie, once the rules of the game are well established.)
What the movie's doing, really, is trying to remove from sight anything that would cause you to disbelieve in its core premise: that musicianship is simply a matter of hard work. There's a great deal of talk about genius in this movie, but not until the very end is there even the tiniest hint of what musical talent is like. Fletcher chastises musicians for being out of tune, for being out of time, and nothing else, and outside of abusing his students, all he has to offer them is practice.
Do you become a genius by playing until your hands bleed? Of course not. That kind of lunacy is orthogonal to the question of genius. But blood and sweat and grimaces are all visible signs of struggle, and the movie is all about struggle.
My point here isn't that Whiplash isn't great. It's a fantastically entertaining and exciting movie. But it achieves that by cutting away everything that doesn't matter. And that includes the way things work in the real world. In the real world, these two characters are not only horrible sociopaths, they're boring beyond belief. But keep them pinched inside a powerful enough magnetic field and they're incandescant.