It's so tough as the stepmom to have a role in helping direct your stepdaughter. In high school, I also lived with my dad and step mom, and I can assure you that what I wanted more than anything was for my dad to take time out of his day to spend with me. Men tend to assume that because they just wanted to be left alone as teenagers, that their daughters are the same way. I really wanted him to be interested in what was going on in my life. I wasn't a bad kid in school, but I did mess up a few times, and I think that had we been closer, then it would have been a lot easier for me to make the right decisions. It's so important, as a teenager, to have open communication with your parents. It relieves so much tension.
You guys need to allow her to tell you how she is feeling about home life and school life. She needs to feel as if she can vent her frustrations (without being rude, of course) without you guys getting upset with her. This will open the door for her to bring problems and situations to you guys that she is dealing with in her life.
If you guys go to church, be sure to get her involved in the youth group. Church is such an awesome place to find positive friends and influences.
I know most of this was from my experience as a teenager, but I hope it helps you out!
-A.