Hi Garfield. I'm not sure if you'll want to listen to my advice or not, considering I'm ten years older than you, but at least read my response.
I weighed about the same as you do when I was 13. And yes...I remember feeling that I was the biggest pig in the school. But let me tell you, these days, when I look back at those photos, they bring me to tears because I actually had just the cutest body. At 126 I had great curves and wasn't in danger of looking anorexic or of looking obese. It was the perfect weight for my age. But I do remember crying just about every night because I felt so terribly fat.
As a 23-year-old, looking back at what it was like to be 13 years old, I can tell you confidently that girls (unfortunately the age group doesn't seem to matter much) often become extremely jealous of other girls and will say things about each other and spread rumors like wildfire. You might have the kind of body they wish they could have. Maybe they all still look like 10-year-old boys (you know...flat and straight without any hips or curves to speak of). Everyone has insecurities and sometimes people lash out at others and pick on their weaknesses because it's something they're insecure about themselves. Maybe these people who are telling you these unkind things are insecure about their own weight or eating habits.
Sweetheart, at 13 you do not need to be thinking about how much weight you should lose. At 13 you need to be thinking about your crushes at school and the great outfits you can wear and enjoying your time as a budding teenager.
It is very difficult to change the way we see ourselves in the mirror. It has taken me ten years to finally understand that concept. But that change can be made; it's just a slow process. In the meantime, you need to learn how to rewire your thoughts so that you're providing positive feedback. Think about your wonderful strengths and attributes (are you an awesome friend? an amazing student? do you have a great smile? are you loving? kind? strong of mind and spirit?); focus on finding those terrific parts of your true Self. Reevaluate who the people are who call you fat. Are they truly people you want to be listening to? My feeling is this: if the people who are concerned about my weight are not my physician, then I couldn't care any less about what they have to say about it. If your physician has been voicing concerns about your weight (which s/he shouldn't be at this point), then you should listen. Otherwise, forget about them. Trust me.
You are not responsible for another person's insecurities or jealousy. The only person you can accept responsibility for is yourself. It seems to me that you need to sit down and make an honest list about the wonderful parts of your true Self and focus on that list to improve your confidence.
Have a good night,