Very common problem with the girls of this age. Starting in the pre-teen years, for the girls, the body becomes an all consuming passion and barometer of worth. Self-esteem becomes too closely tied to physical attributes.
 
Here is a very good resource for how you can help your daughter develop healthy self-esteem:

Sounds like outlash from being bullied herself. Age 12 is kind of difficult but one thing I would do is maybe take her shopping and give her a certain amount of control over what she chooses. This will help her in making choices and then feeling good about herself. Plus she has control and is not having to be controlled by bullies. Next, I would have a meeting with the school and then send them a letter requesting help as well. A letter is usually not ignored. Finally, I would talk to your daughter about self image. Sit down and ask her to give you 5 good things about herself to you. Then after this, have her meditate on her favorite. All human beings are made of both weaknesses and strengths. How can she strengthen her weaknesses? Sometimes a diary will also help.
Good Luck