Initially a mere understudy in Fletcher's prestigious Shaffer Studio Band, Andrew gets a big "break" when he seemingly misplaces a folder.
Primary drummer Carl Tanner (Nate Lang) puts Andrew in charge of holding his sheet music, but when Andrew puts it down for a second to use a vending machine, it mysteriously disappears. Since Carl doesn't have the band's entire routine memorized but Andrew does, he takes over the role of "core drummer" for the performance that night and moving forward.
Reading between the lines, it seems like Fletcher is most likely the person who stole the folder, even if he does put on a good act of outrage as he becomes aware of its disappearance. Without context on its significance, it seems very unlikely that a stranger or janitor would pick it up in the brief moment Andrew leaves it unattended.
Based on the driving motivation to prove himself by his hard work and talent, Andrew didn't pretend to lose it, either. Late in the film, when Andrew rejoins Fletcher after his firing for the JVC performance, he's using sheet music for "Whiplash" that have the exact same "loud" and "softer" hand-written notes that were seen on Carl's sheets earlier. While it's not explicitly confirmed, Fletcher might have given Andrew the very same missing sheet music.
