Camp Scare by Delilah S. Dawson

An eerie, twisty ghost story about twelve-year-old Parker, who only wants a summer of fun and new friendship at sleepaway camp but ends up finding a nightmare instead!
Don't forget your flashlight. . . .
Parker Nelson can't wait for summer camp. She'll have fun and make amazing memories, far away from the bullies who made seventh grade unbearable.
But then something terrible happens: The mean girl who made life a living nightmare is in Parker's cabin. Soon all the other girls turn on Parker, too-no one wants to be her friend. Except Jenny.
Jenny's the only one who is willing to listen. The only one who understands. The only one who feels the same way Parker does: that there's a deep, dark secret to making friends and she's the only one who doesn't know it.
But there's something else Parker doesn't know. Something bad happened at the camp a long time ago, and it just won't stay buried. . . .