You're young and so are they. She probably doesn't even know what stuttering is. While it's hard, the best thing to do in this situation is to tell her you have a stuttering problem. Yes, they will initially feel bad, but then they will understand you and what is going on next time you speak together. I've noticed that the older the person is, the more understanding they tend to be. And once you get into college, students are a lot more understanding, and for the most part don't care, but you'll still need to advertise. Sometimes you don't need to though. Some people are smart and can figure it out; normally only older people though.