Being a teenager is really hard. As a mom it's natural to want to protect her, but really this may actually be good for her. Learning to take care of herself and dealing with jerks are 2 of the best lessons a girl can learn. The best thing you can do is support her and offer help, but let her handle things on her own where she can.
There is not much you can do to build her self-esteem. By that I mean that most teenagers have low opinions of themselves and they are the only ones who can change it. No matter how much you tell them they are smart or beautiful, they won't believe it until they are ready to.
What you can do is remind her that these boys are only teasing her to mask their own feelings of insecurity and because it provokes a reaction out of her. Unfortunately, a lot of my students do this, and they always focus their jabs at whomever will get the most upset.
One strategy I tell my kids is that when someone teases them, is to calmly respond, "How sad that you feel so badly about yourself that you have to try to put other people down." You'd be surprised at how often this completely knocks them off track. She should also practice 'confidence'.... I tell my kids that no matter how ugly, fat or dumb they feel,that if they act like they are completely confident and feel good about themselves, they will find that others will see them that way, and they soon will too.
It worked wonders for me in school. I was (and still am) a larger woman, and would always get picked on, but once I started saying things like, "You're just jealous because I have curves." or "You must really like me to be looking at me so much" I found that the teasing almost completely stopped, and what little still happened I was able to easily shrug off, because I actually started to feel confident. Have her take some time each morning to tell herself what a great girl she is, and maybe write a list of all the great things about herself to look at when she starts to feel badly. Sometimes we forget just how amazing we are.
There are a lot of 'curvy' girls on my campus, but it's interesting to see that the only ones who get picked on are the ones who seem to be ashamed of it. I have one girl in my class who would be classified as obese, but I have never heard anyone make fun of her or call her fat. In fact, many of the boys think she's 'hot', simply because she is confident and smiles a lot. She doesn't cover up (nor dress like a tramp).... she just wears clothes that flatter her figure and seems comfortable in them.
Interestingly, the girl that does get picked on is far slimmer, but tends to wear clothes that cover her up and often looks down and doesn't talk to her classmates.
Help her to think of a few good responses and pratice them until she feels confident with them. I don't know how nasty you want to get, but one of my favorites is turning the tables.... like... if they were to ask when she's expecting, I might respond..... "expecting you to grow a brain? I think I've given up on that one." Or about the necks..... "more than how many brain cells you have left." I know it's a little mean, but honestly, high school really is a jungle, and sometimes you have to go for the jugular.
Hope that helps! Good luck!