Who took the folder in the movie Whiplash? Let' pour our minds.

Andrew knew the whiplash by heart, but his character is shown that of a honest person. Fletcher is unpredictive and I don't know if he would do so by risking his own show. I mean he didn't know that Andrew memorised the music by heart.

It doesn't seem that Tanner would do that by himself. And who is Janitor? I searched for answer in Quora, but people are just guessing about Fletcher.

Maybe Tanner wanted himself out, maybe Andrew did so subconsciously, maybe it was Fletcher's way of getting them tested, just to get another chance to scream over them? What do you say? Who took the folder?