Perfect Little Monsters by Cindy R. X. He

Someone has murdered the queen bee of Sierton High School. All the dead girl's friends are suspects. And each one has a reason for wanting her to die.
Ella Moore was the most popular girl in school.and also the most hated. When she's murdered at her own party, there are too many suspects to count. And too many people who think she deserved it.
The police's prime suspect is the new girl, Dawn Foster. Dawn was the last to hand Ella a drink on the night she died. Plus, all of Ella's friends with a motive for wanting Ella dead are more than willing to throw Dawn under the bus, if it means keeping the heat off themselves.
But Dawn refuses to go down without a fight. She's determined to clear her name. As she delves deeper into the past, she discovers that Ella and her friends had major enemies, and someone is out for revenge. Dawn must uncover the truth before the police arrest the wrong suspect. and before the next person dies.