You could reject her. You could walk away when she talks to you, make rude comments about her and let it be known, "I don't like her". Or, you could ignore her. You could make it a point to act like she doesn't exist. When you walk by her desk, you keep looking at the cooler; when you see her on the street, you pretend like you don't; and when you're in class, you sit far away. You act like she someone you don't know.
An article in the Journal of Personality and Social psychology, shows that many times there's a difference in the kind of focus created in each method, whether you reject or ignore Cindy. It is likely that if you reject her, she will develop a prevention-focus. In other words, she's going to prevent feeling rejected--like by avoiding you. You don't want this to happen (though you really should) so you're left with only one option: Ignore her.
But, ignoring her isn't just your last options; it works. According to the article in the Journal of Personality and Social Psychology, if you ignore her, you create a much different focus. Instead of making her want to escape the pain of rejection, you are likely to create something called a promotion-focus. A promotion-focus is obvious, it will make her feel the need to socialize with you. She will focus on promoting your relationship.
Out of the all the options--nice, reject or ignore--if nice doesn't workout, then: Ignore. Just be careful, don't take the ignoring too far or it becomes rejection. So if she tries to be nice, give her a little nice back. Keep a little mystery though.