I suspect (her friends) for some reason don't like you or they don't believe you are "in their league". However this girl is attracted to you or thinks you are nice. This puts her between a rock and a hard place. (She does not want to look like a loser to her friends) and yet she can't help but acknowledge she is attracted to you (when they aren't around). Bascially she does not want her friends to make fun of her for liking you.
Since this clearly means the girl lets peer pressure dictate her behavior you have only two options. Either you can play along with the hope of having a "secret relationship" or you can ignore her and move onto a girl who does not listen to her friends when it comes to who she should date or be attracted to.