Wait; the average length of a high school relationship is 5 months. If patience is not a virtue, the likelihood is that in 5 months, you won't feel the same about her anyway. If you do, it's a good sign.
You just have to treat her well so that you can continue being the good friends that you already are. However, this will also depend on how you view your friendship since she already has a boyfriend, because it is not good to cause a separation between two people.