How can I stop caring what people think and just be myself?

I'm 16 and a junior. With family, it doesn't matter if it's at home or in public, I'm this awesome, dynamic person (or so I think) who makes people laugh and dances and basically just does whatever. But at school, I don't know why, I just get all tense and can't loosen up. I can't even talk to people sometimes because I get all tongue-tied. I find myself talking in this barely audible monotone, even with my friends. I can't even laugh! I feel so self-conscious and every time I go to school I tell myself, "Hey, today I'll be the way I am with my family!" but for some reason I just can't do it. Please help me. Thanks.