Try ..The Clique Series by Lisi Harrison.
The first one is The Clique a Novel.
Claire Lyons moves with her parents from Florida to wealthy Westchester County, NY. Until they can get settled, the family stays in the guest house of Mr. Lyons's college buddy, who happens to have a daughter who is also in seventh grade. Expected to welcome her, Massie instead chooses to make Claire's life miserable for no other reason than she's the new girl. Massie enlists her clique of friends at Octavian Country Day School, all part of the beautiful and popular crowd, to help with the harassment, which ranges from catty comments on Claire's clothes to spilling red paint on her white jeans in a conspicuous spot. Tired of it all, Claire tries to fight back, but then the abuse worsens.
Another series you to try is Private…the first one in that series is called
Private by Kate Brian.
Reed Brennan is nervous as she begins her sophomore year as a transfer student at prestigious Easton Academy. All of the other students are richer than she could ever believe, and she immediately finds that she's in way over her head academically. She determines that she'll do anything to keep from flunking out and going back to her dreary home. Reed aspires to be just like the beautiful and brilliant girls who live in the luxurious Billings dorm, but she has little hope of ever being accepted into that elite crowd. Somehow, though, she edges in and barely notices their cruelty because she's so grateful to be tossed the crumbs of their kindness.
Lock and Key by Sarah Dessen.
Ruby hasn’t had much success with family. Her father left; her protective older sister, Cora, left; and her boozing mother finally leaves, too. Ruby is alone until Cora learns of her situation and swoops in. Suddenly, Ruby finds herself living with Cora and her wealthy brother-in-law, attending private school, and wondering just where she fits in. As in previous books, Dessen takes on a central theme—here the meaning of family—and spins many plots and subplots around it. Most prominent is the thread about Cora’s relationship with boy-next-door Nate, who rescues her when she needs it, but has difficulty accepting Ruby’s help, tentative at first, when she discovers he’s being physically abused. Nate seems too good to be true.