Heck yes! Confidence is DEFINITELY possible. I'm thirteen, and I know exactly what you mean. I used to have really low self-esteem, but I figured out how to get over it: raise it up. I used to hate everything about myself, but then I thought, "Is this rational? I'm beating up on myself WHY? I'm pretty in my own way, I have friends, and I know people love me for who I am." Just tell yourself about what really matters - your personality.
I'm not popular, and don't think I'll ever be, and that's fine. Why should we change ourselves to please others? We shouldn't! Be confident in yourself because you're a good person and you're willing to let that show. If you are confident that you are loved, you will shine. If you knew me a year ago, then met me now, you would think I had tricked you by showing you my secret twin or something. I was sad indeed. I didn't look people in the eye, I spoke quietly and only when spoken to, and acted as if I was afraid that someone would come out of nowhere and tell me how terrible and pathetic I was. And that's exactly what my fear was. Not clowns, spiders, pain. I had such low self-esteem that I thought everyone hated me.
I finally gave up on trying to be invisible. "I'm sick of this," I told myself, and (I seriously think I freaked everyone out with this) started to talk to people, lift my eyes from the ground, and care less about what everyone thought. It took a while, but eventually I became practiclly immune to the fact that maybe someone thought I looked funny, or I wasn't wearing the most expensive shoes. "Who cares?" I thought. "Not me." And you shouldn't, either. Now, I'm not saying to show up to school wearing crap - that would sink your self-esteem lower. I'm saying to spend less time on what people think, continue to be your kind self, and smile at the world. Confidence isn't that hard to achieve - you just have to take it a step at a time. : )