All three of my daughters did the smoking thing ... and it wasn't just cigarettes. Neither my wife nor I smoke (cigarettes or marijuana) nor have we ever. Now, realize right up front that I don't put this as if there is anything better in us because of this, but that this is the environment in which the girls were raised. Years later, only the youngest still smokes (and fights with herself for a nasty habit she knows and claims to be destructive to her health). The other two have grown to hate smoking and cigarette smoke. There were times in the early stages where we would make a big deal over it, but it surely didn't help matters. They already knew what we thought of it all, but also knew that we didn't pick our friends based on who smoked or not, for quite a few of them DID smoke. I personally hate the habit, but then again, I drink my coffee every morning, which depending on your micro-culture may be considered better or worse. :) Yeah, I know tar and nicotine can cause cancer and other ailments (and for these reasons am especially glad that I didn't get hooked on it), but coffee has the nasty habit of leaching the calcium right out of your bones so that you're taking the chance that your bones may simply snap as you get older. I say all this to put a perspective upon an easily narrow-minded view - believe me, I know that all too well. :)
Signs of rebellion. I would rather have these signs be more obvious than to have them hidden as is the habit of so many (which is probably what happened when you discovered this hidden rebellion in your daughter). Now, I know it doesn't cause your reputation to fare well by those who judge according to appearance - especially in the church -for most of our church leaders are living in a fantasy world where they have been made leaders because they have their stuff together better than their peers.
The really important prospect before you is to find out what is REALLY going on with your daughter! Don't let your fears of appearing inadequate as a parent force you to deal with your daughter outside the reality of relationship! You will be pressured to make her conform ... but conform to what? By telling me that she is a believer in Christ I can only guess that there are specific standards that you hope she live up to. I don't say this in a way to suggest anything goes but instead to stir you to consider what it is that you expect her to be like as a believer in Christ.
What is it that you hope she WON'T do that you may have done, or what is it that you hope she WILL do that you did not do? Or maybe, even, what is it you hope she will do that you did? In other words, are you hoping she will avoid most of the inadequacies yourself, or others, experienced - or seem to be experiencing - at her age? Would you rather her be more like how some of the other female believers her age APPEAR? I have lived long enough to discover that things are not as they often appear, and that MANY who came across as being so spiritual eventually were realized to simply be playing the game. I have learned this through MUCH disappointment.
Though you will become convinced many times that you have so little input in her life I tell you that NO ONE is as important to her as YOU are!! It won't be your words (though don't be afraid to talk to her because of this, and believe me, she hears a lot more of what you say than what you will ever think) or your incredible insights, but it will have been YOU YOURSELF. A father is an incredible thing to be to another person!! First, and foremost, be her DAD. Love her for who she IS, not who it is you would like her to be, and definitely NOT what it is someone else thinks you ought to make her to be.
Why do you think your daughter is depressed, did she say why? I mean, I would be surprised to find very many girls her age to NOT be depressed (mine were, and still often are). The fact that she has admitted to this is a good thing, for it is much better to realize it so you can be sensitive to her in this. Depression often goes hand-in-hand with disappointment, that is, not having needs being met. The needs are not usually the things projected and claimed as being needs, it is more basic than that. Whatever sense of lack we grew up with has only been increased in our society as we have put many more things in that category called Necessities! True need is that void within us from having been originally made in God's image but having also continually attempted to fill this void by the multi-faceted characteristics of EMPTINESS. In other words, because we keep drinking out of an empty cup that APPEARS to contain a substance and we will jump from one cup to another hoping to finally get filled.
Grades have become like gods to us. What is your real concern in knowing that her grades have slipped? The fact that her grades have fallen simply points to the fact that she has become distracted. Not unusual for a 15 year old girl - especially considering her situation with a mom and a dad who don't live together anymore. I'm not rebuking you in any way, just adding it all together to give a fuller picture of her world. Once again, consider this: grades vs. relationship. The picture will always become clear when you can see what you're dealing with. :)
I personally think it is wise on your part to not buy her a car - for what would be the real purpose of it? Don't let the temporary sense that "Daddy loves me because he bought me a car!" fool you into thinking it will do anything in the long run as far as showing your daughter that you love her.
I appreciate your concern for your daughter, and thank you for taking the chance to ask me what you did. I look forward to continuing our discussion. :)