Cheerleader Camp
(1987)

All of the members of Alison's cheerleading team are killed off. Alison (Betsy Russell), who has been feeling mentally unstable under the pressure of the cheerleading competition and thanks to tension with her philandering boyfriend Brent (Leif Garrett), gets the blame for it all. 

As she is being taken away, Alison puts two and two together and realizes that Cory (Lucinda Dickey), the team's mascot and Alison's confidante, was really behind it all.  Cory was jealous of the cheerleaders and resented the degradations of being a mere mascot when what she really craved was to be one of them. 

Now, with Alison and the others out of the way, she can realize her dreams, and as Alison is driven away to either jail or a mental institution, Cory is seen happily practicing a routine in a cheerleader costume.  No one believes Alison when she blames Cory for the murders because they all think she is crazy.
 

Thanks Greg B!