Girls Under Pressure — Jacqueline Wilson

Ellie thinks she looks awful. Horrible. FAT. Her best friends are both drop-dead gorgeous and Ellie's sick of being the ugly duckling. So she goes on a diet. And she even starts to exercise, much to her friends' and her gym teacher's amazement. Ellie's hungry all the time, she works out every spare second, and she's turned into a grouchy meanie. But if her friends don't want to deal with the new and improved Ellie, that's their problem. It's better to be thin than happy. Isn't it?