How can I be myself, more next year?

At school I always feel trapped, in a shell, like I'm never myself, I mean its because I'm shy... but I want to see how I can be more outgoing and talkative, at school I'm like a completely different person, I'm shy and quiet it bugs me because I'm only like that there. I have friends, quite popular ones actually, but I still feel trapped, like last year I stopped myself from joining Theatre Arts because it was onstage I can sing and act, but no one even knows that because I'm not open. I don't know whats wrong. I just want to be ME, this year.