How can I become more self-confident?!

Around my friends, I'm generally very outgoing and chatty, but around new people I'm terribly shy and self-conscious. Growing up, I was a really weird child, and though I make friends pretty easily now-of-days, I still find it hard to talk to strangers- especially in Spanish, which is my only non-honors class. I feel as though I'm being judged when I speak (I have a passion for languages and work really hard to perfect myself in that class), and it's really embarrassing. :/ There's a group of girls that sit by me in that class, and I hate to judge people, but they're all very shallow. Whenever I say something to somebody nearby, they look at me like I'm doing them some kind of offense, just by opening my mouth to speak. I'm not a loudmouth in class (I'm actually very softspoken), so why would they be giving me such dirty looks? I've even known some of them since elementary school. Really, I'm disappointed in myself that I'm letting their opinion get to me, because they're such annoying girls (the only thing they talk about is how "smart" and "beautiful" they are, when in actuality, they're neither). How can I learn to be myself, even when around strangers and judgmental people?