When I was younger-- in the early years of puberty, between like 5th and 9th grade, everybody would tell me that I was ugly. Everybody said that I was fat and ugly and that I had gross skin and hair, and a flat chest.
Now I'm 18 years old and I've stopped growing pretty much. I'm still overweight, but I've got a nice figure and I'm frequently described as 'beautiful' or 'gorgeous' by my peers. I'm described as having beautiful skin and cute hair.
Why? I'm not really sure. I haven't changed that much since when I was younger-- it just took me longer than most to "grow" into my own body-- for everything to settle and look good together instead of looking strange.
If you're young-- even if you're not too young, you still may just be taking a while to grow into your own looks. It's like the story of the ugly duckling that turned into the swan.
Don't let those jerks make you think you're ugly. I'm sure they're wrong-- every girl is beautiful in a way, I think. I've never found a girl that I couldn't see something beautiful in. Maybe the shape of her eyes or the freckles on her cheeks or the way their hair looks on a windy day-- not everybody has the same kind of beauty, and that's perfectly fine.
Look at yourself and pick out what *YOU* think is prettiest about you, and show it off. Even if some complete retards make fun of you, they're most likely just jealous. Even if you don't believe me now, trust me on this one. You're making them jealous somehow and they're trying to break you down by calling you ugly, since they know you have low self-esteem.
Screw those jerks, believe in yourself and find your own sense of beauty.
Having a beautiful personality also makes you more attractive too, and those jerks will always be ugly in that sense.