advice..
forget about what anyone else thinks. at the end of the day 6th form is going to be nowhere near as bad as the real world, and if you don't develop confidence to even tell the teacher that you don't understand something, in the nicest way possible you won't get anywhere!
if you have potential, don't be scared to speak out. what will you lose..? ask yourself, whats the worst that could happen if you spoke a bit more? if anything people will talk to you more back, its really intimdating for me to sit next to a really quiet person because we have nothing to talk about. i'm sure most people would feel like that around you.
being shy's a lot worse than being confident, but not as bad as overconfident.
i'm sure you're a lovely person, we all are, but until you show people your personality everyone will just think you're a quiet person.
stop thinking about what others may think of you, you're now in 6th form and these evidently could be the most important time of your educational life!! don't let yourself hide in the corner, express yourself!
im rubbish at giving any good advice, if anything i come across as blunt, but its a hard world...