The purpose of touching someone's arm is to make her feel trust and be comfortable. Of course, smiling is a must to make her feel comfortable. She needs to empathize with you, and touching her arm is something you do in a date, when you greet her, not in class. Muster the courage to do these little things. Women interpret such moves as friendliness, as long as you keep a straight face. There's nothing wrong with being nervous.
Just to give you some examples of other people growing in their social conquests: Search
'seduction school' in YouTube and watch the 5-part program aired in Britain's Channel4. http://www.youtube.com/watch?v=9DYn4-h3Cnk It gives good basic insight on how to first approach women.