No, it is never OK.

Rating: 1/10 (7 December 2018)

In this film, a music teacher drives a young man to commit suicide (but that's OK because he was fat and talentless). This same teacher is later vindicated - a few moments before they roll credits - because his violent approach to education has finally enabled a great musician to shine...
...so it was all worth it. One (non-fatal) car crash, one suicide. One student quit playing drums. Worth it... because "a great" was born. And isn't that how ALL great musicians are born? Oh, obviously. Could you ever become a talented person WITHOUT a violent maniac "sacrificing his own humanity to push you to your limits" (copied from 10 star review)? Obviously not.
Or maybe... just maybe... Whiplash is based on a lie which is tempting to believe: that "greatness doesn't come easy" so you have to be "cruel to be kind". Is it all right to buy into this cliché nonsense just because a film is shot beautifully? Nope. Because the music is exceptional (and it is)? Still, no. It isn't. A young man commit suicide because of a teacher who ignored his duty of care. This really happens. Real people die. No, it is never OK.
Whiplash shamefully missed the point. Film awards come cheap nowadays but good teaching skills are hard won. Any idiot can scream and shout. Terrence Fletcher was just a rookie teacher who liked jazz music but failed practically every student who ever came his way. And he failed Andrew, too. In many countries, people like Fletcher do not simply lose their jobs. They go to prison. Which is a good thing. A really really good thing.