Part of Your Nightmare (Disney Chills, Book One)

Eleven-year-old Shelly Anderson just wants to be popular. Her parents have split and she has to start over at a new school with different classes, a brand new swim team and an unfamiliar social scene. So what if she just wants the cool kids to like her? Is that really too much to ask? So when Shelly finds a mysterious nautilus shell that summons the infamous sea witch, Ursula, she jumps at her chance to make a deal that will solidify her as one of the coolest girls in school. But when Shelly's wish quickly goes belly-up, she must figure out how to back out of the witch's deal before it forever binds her fate.