A boy walks into school, stops in the bathroom, and what follows is a shooting down the main corridor. Then the bell rings and he wakes up — he has been sitting in a classroom the whole time. He reaches for his backpack, and the film ends before we see what he takes out of it.
The two shots fired in the bathroom happen off-camera. Nothing is shown being fired and nobody is shown falling, which is what leaves room for the ending to reframe everything before it as something he imagined rather than something that happened.