How is Drama for GCSE if you are shy and not that great of an actor ? Will it be great to help improve one's confidence and public speaking? Or if you are not good an actor than it is simply not for you.
If you're concerned about confidence though then I'd really recommend joing a youth theatre group or a little theatre - google youth theatres and amatuer dramatic societies in your area (am dram is usually cheaper) and go along - they're very friendly and welcoming to new people as a rule so if you're scared don't worry you'll be ok...
Seriously no drama kid worth their tap shoes will be mean to you at a drama class - it's just not done.