Who is Stacey from Hate List and what is their importance?

Stacey and Valerie were best friends since kindergarten. After the shootings, Stacey came to the belief that Valerie had some prior knowledge of the shootings, and that she could have done something to prevent them. Stacey is friendly toward Valerie when school begins again, but does nothing to include Valerie in with the rest of their friends.