The "guards" in the woods are actually 3 other outcasts from school (seen on the school stairs and in the cafeteria). They are not the same outcasts as the ones in the party house, even though they are wearing the same costumes. This was meant to confuse the police when it was all over. Due to time constraints, this subplot was cut from the film, and is only subtly explained. See more
[while Emily spreads a substance on Heather's face
What are you doing to her?
It's a special compound we made up. At first, she won't feel anything but a cold sensation, then a slight tingle, then a mild burn. Soon enough, it will begin to eat away at her flesh, dissolving it ever so slowly, in the most painful way imaginable, and she will become outside what she is on the inside: disgusting.