Of course it is, unless there is a law where you are that sex below the age of consent is OK if you aren't too far apart in age (often called a Romeo and Juliet law). On the face of it, you're both below the age of consent (18 in Arizona) so you would both be in trouble. (Being a minor is irrelevant - it's only the age of consent that counts. It just happens that both are the same age in Arizonan law.)
HOWEVER, you certainly fit into section 13-1407-F in Arizona law. This says "It is a defense to a prosecution pursuant to section 13-1405 if the victim is fifteen, sixteen or seventeen years of age, the defendant is under nineteen years of age or attending high school and is no more than twenty-four months older than the victim and the conduct is consensual."
To put this in plain English and work it through, you're under 19, check. She is over 15, check. You aren't more than 24 months older than her, check. Your sex will be consensual because you WANT to do it, check. Everything fits into what that section says. So you have a total defence to a charge of underage sex.
It's not exactly an exemption, but it amounts to the same thing. If the DA would be daft enough to charge you with statutory rape, all your lawyer has to do is say all this in court and the judge says "case dismissed!" So a sensible DA just wouldn't charge you in the first place. If I can tell you all this, a DA ought to know it even better.
Just be careful and do the right things - you really don't want her having a baby when both of you are still in school and can't look after it.
BTW do you know the Shakespeare play "Romeo and Juliet"? That's where "Romeo and Juliet law" comes from - they were young lovers, kind of like you two. What makes a play out of it is that their families hated each other, so they had to be secret about their love. "West Side Story" is the American musical version!