Be sure to have her checked medically, to insure that there is not some irritation or inflammation that is making her uncomfortable. This is unlikely, given the long-standing pattern you have described. However, be sure to check nonetheless. It is normal for all children to have a way to soothe themselves on the one hand, and to reduce tension, on the other hand. Her behavior likely serves both purposes. There is no magic way to talk about this. You've probably said all there is to say about it - i.e., it's OK in private, it's not OK in public. She is old to be doing this in a public setting, and it's likely become a habit pattern with her. I would arrange a mental health consult because you want to rule out that there is any compulsive aspect to this behavior. It is sometimes seen as a manifestation of obsessive-compulsive disorder. I am not suggesting this is likely, but it should be considered.