The best advice I can give you is a quote from a song...
"If today was your last day, and tomorrow was to late, could you say goodbye to yesterday?"
As in, if you died today would you be totally content with what you did? Live each day like it's your last you know? You may think well it's only school, etc. However, why be withdrawn instead of outgoing and making other people happy. You're dissapointing yourself it sounds like, as well as the other people that could be laughing at what you say/do, having their day brightened by you, etc.
U.S. Schools...A LOT of kids can be complete assholes. However, you just have to look past them. You will meet a few people that you know you can trust and come to about everything, however there'll always be people trying to put you down and take away your joy. Just keep your head up, and lead the life you want to live.