Tell girl i like her without risking friendship?

There's this girl that i met in one of my college classes. We are both first years, have been sitting together since early September, and I've really grown to like her. During class we always talk (i know... we souldn't in class), we always have fun conversations, and we talk on MSN all the time. I want to ask her out but i don't know if she feels the same about me. How can i ask her without ruining a great friendship?? I can't tell if she likes me b/c the signals are mixed and i'm no good at pcking up girl's signals. Sometimes in class she touches me. Not in a sexual flirting way, but just as if we've become good friends and she's comfortable with me. Like she might randomly grab my arm and bury her head in it to show she's tired. We text msg too. But we haven't really done anything together outside of class even though she said she would have lunch with me and my friends sometime. It's weird b/c i have another good friend (a girl) who i met this year and we have coffee, hang out, and

Tell girl i like her without risking friendship?
Guess what? She already knows that you like her and she is telling all of her friends about you because you spend so much time talking and women love a man that has some good conversation! Take her up on that lunch offer one day and see where things go from there! Don't spill the beans just yet, but don't wait too long or someone else will put in their application and snatch your friend away! Like I stated before nine out of ten times she knows you like her and she probably likes you because she spends so much time talking to you, women don't give of their time so freely to someone they aren't interested in!

Dancing