Good grief sethrp-1, you COMPLETELY missed the point. The girl was only seen briefly specifically BECAUSE she was the one who was going to kill herself...everyone else was so wrapped up in their own stories they didn't notice her, nor did we. As one of the other students says at the end - we're all so wrapped up in our own problems we don't notice what's going on for someone else.
The director himself said if he had killed off one of the others, it would've suggested their problems were worse than someone else's. The whole point of killing Kelly was that she was unnoticed by all of us. Get it now??