Though its believed that Megan is the killer and she is getting revenge on those who threw her down the shaft at the beginning of the movie, the killer is actually Cassidy's boyfriend, the Valedictorian. He decided to kill everyone who knew about what happened except for his girlfriend because no one was keeping secrets very well. One person told another person, and that person told another, and so on, but alas the secret was never told to the authorities.
The valedictorian wanted to make sure that no one talked so he and Cassidy could live happily ever after.
The only three people to survive are Cassidy, Ellie, and Megans little sister.
At the end of the movie it is implied that chugs' little brother Garrett, who dated Megan and accidentally killed her, is still alive after slitting his wrists and getting run over by an Escalade, and that me might be doing some killing in a sequel.