Background: My father was an alcoholic and two of my brothers were into drugs. No dependencies in my husband's family. I had discussed the issues of genetics with my daughter and cautioned her about alcohol and drug use. She's always felt as though she doesn't fit in and does not have a lot of friends; however, she's had one good friend since kindergarten and another close friend for three years.
As to her not following through on her homework assignments and lowering her grades in the process, you can, as always, state your expectations of her working to her maximum potential and continue to encourage her for any progress she makes in the homework area. Volunteer to do anything you can to create the best learning and studying environment possible at home. She will learn the natural consequences of not doing work and then have to deal with how that makes her feel.
Right now she probably thinks you think she has become a kid you don't like or trust. You have to avoid preaching and making her feel this way. You can express your concerns without abandoning your belief in her. If her depression goes on for weeks and she is withdrawing from friends and family, I would consider you go see a family therapist and get some good advice. Teens' moods change radically at this stage from day to day and yes, she will be defensive and react as if she is under attack. Abide with her, support her and live your values. Good luck.