[Eternal]

My best advice is to not have sex and instead spend more time going out on adventures and activities, at least until she is 16. In my opinion 15 is a little young to be having sexual relations anyway. You're both young and while you have time, you should go out and have experiences. Go on dates to museums, go on a picnic at the beach, take a pottery class, etc. I dated a boy when I was 15 and my regret about it is that we just stayed inside and watched movies whenever we hung out. I wish that instead we had gone out and done fun things together. It will let you and your girlfriend get to know each other even better too!
Going back to your question though, I suggest playing this safe because if her parents find out and aren't aware you two are sexually active, they could get you into trouble. It's better to just wait, there's other sexual acts you two can do besides intercourse if you want that intimacy factor.