While I love the plotline, and I think you've got a great idea, the writing is not-so-great. But I've been writing and critiquing for a long time, so I kind of hard to please.
There is one things that this story needs to be very, very good: it needs more detail. Describe exactly how Gabriella felt when she realized the killer was after her. Describe how Sharpay was annoyed after Gabriella decided to read instead of talk on the bus ride to Alpine Camp.
Detail will make the story more interesting to the reader. Also, it will make the flow better. With all of your dialogue, it's a little choppy.
Please don't take this the wrong way. I'm a tough critique, and I know what I like. You have a suspenseful, original plotline. I've surfed through the HSM section on FF.Net, and even there, there isn't a story like yours. Props for originality. But with a little effort, this story could be really good.
As a fellow writer, I wish you the best with your writing.