I appreciate Jennesque's advice. There's something to be said for getting along, even if you have to fake it.
But as Shakespeare said, ..."To thine own self be true."
It's really nobody's business what you believe, only how you behave. Learn to change the subject or insist on others' respecting your privacy. Get mysterious, they love that. Don't discuss it at all, with anyone.
Read Richard Dawkins' The God Illusion. or anything by him. Find an athiest discussion forum that you like, people with your point of view, for encouragement and solace.
Hang in until you're eighteen. Then show them your back.
I totally agree that you should be true to yourself, but not everyone wants to stick out like a sore thumb, and that's something that may happen to an atheist at a Christian school. Personally, I'm a very introverted person. I like to keep to myself for the most part, and I don't like people gossiping about me. I don't care what they think, but I also did not want the entirety of my 7th and later 8th grade class, teachers, pastors, classmates parents, etc talking about how I lived my life. I don't like to call attention to myself. I'm not sure if the OP is like that at all, but that is why I chose what I felt was the most subtle route I had to show I wasn't a true believer, because the people at my school were always in everyone else's business. It's only because they care about what they believe to be the welfare of your soul.. they're not doing it to be cruel.
It might be hard to lie to yourself, but sometimes being honest about something, at least in this case, can be more difficult. Being young, it might be best to just go with the flow as best you can tolerate it, unless you're an adult and free to live your life as you please. Throughout middle school and high school I always told myself that nothing really mattered except my grades and what I learned. The only people I still see from that time period are the only ones that matter.
I think people al my school would be very accepting except a couple of teachers, but my close circle of friends is what I'm worried about. I know they would accept and probably understand, but I don't know if they would think of me the same, and I'm most worried about little talks from their parents >_< I want to be brave and open, though. Anyone know how I should go about telling people? Who should I tell first?
I would suggest starting with the people you think would be most accepting and trustworthy enough not to tell anyone else. That way you can do a sort of practice run before moving on the tougher nuts to crack.
Your friends, starting with the closest one first or maybe the one you think may agree with you the most. Your friend will obviously know your other friends better than any of us probably ever will so he/she can hopefully provide the best advice on how to proceed.
Good. If at the end of the day, you don't care about the potential of losing your friends, or at least having a change in the relationship you have with your friends, go for it. And I am by no means trying to discourage you.
At the end of the day, the only one who's always 100% going to be there for you is yourself. Just worry about keeping you happy, without trouncing on the hopes and dreams of others of course. :) if someone cannot accept you for who you are, they're not worth your time.