What happens to Melinda at the pep rally? What did she do that makes people do this to her?

A girl behind Melinda at the pep rally accuses Melinda of being the girl who called the police on Kyle Rogers, "Aren't you the one who called the cops at Kyle Rodger's party at the end of the summer?" She pokes at Melinda. Other kids turn on Melinda calling her an "asshole."