The Hex Factor — Harriet Goodwin

Xanthe can't wait to turn thirteen, but as her birthday nears, her world starts to fall apart. Set up at school, it seems her age-old enemy, Kelly, is making trouble for her. With her friends deserting her, and mysterious glowing Xs appearing in front of her eyes, Xanthe turns to her great-grandmother for help. But what the old lady tells her will change Xanthe's life forever...