Hot in the Pot — Lois June Wickstrom

There's a new kid in class, named Squilla Balloo. She's smart. She's popular. She's got to go. There's a book in the library with a magic spell: How to Get Rid of your Enemy. Of course, there's a catch. She needs to get one hair from her enemy. It has to be just one hair. While our heroine is brewing her magic, she invites her enemy to visit -- to obtain that hair. One of her own hairs falls in, too. Now, they must work together to stop the magic before they both disappear.