The flashlight is found in classroom 1-B. Naomi will note that there's a light in this room and will notice a working flashlight next to Masashi Kawasaki's corpse. It is used in the classroom 3-A if you get dragged into the classroom (that is caused by going back and forward through the classroom.). You have to shine the flashlight into the left side of the screen to notice the spirit, otherwise your darkening level will go up and cause a wrong end.
Significant plot details end here.
It's the only item found in the first episode, which is also optional to pick up.