My Theory (Rebecca is a paranoid schizo) ***SPOILERS***
Rebecca is actually a paranoid schizophrenic.
-Charley just simply got high and did something dumb. Even if Ernessa did whisper in her ear and tell her to do it... She was high and out of control.
-Girl in room next to Ernessa's either really did commit suicide, or more likely--she went back on the ledge and slipped. Also, Ernessa didn't go through the window. Rebecca is just nuts.
-The teacher probably got attacked by an animal. (I don't know, best guess I can come up with.) Or Rebecca is so schizo that she killed her without remembering? but that's a far stretch.
-Lucy died of anorexia. It's very obvious. She stopped eating, she became very frail and anti-social, and stopped getting her periods.
-And in the end Rebecca lit the innocent new girl on fire. She sees the ghost Ernessa in the window--all in her head.
This would also explain why Rebecca is the only one who can smell the awful smell coming from Ernessa's room.
Also, Rebecca's dad wasn't mentally stable. Plus both are writers--writers tend to have a few screws loose.
This is just my random theory. (: Being a psych major, I always look at mental disorders as being an explanation in scary movies.