Oh, my You have been given an extremely difficult task. I praise you for your willingness to take it on. If she is with you only on the weekends you are even more hampered in what you can do. She needs consistent boundaries and consequences and you can't give that to her.
I do have first hand experience. I received a foster child, a girl, when she was 6. I adopted her when she was 12. CSD had a psych evaluation done when she was 7 and the psychiatrist said that it is very difficult to change what has been learned during the first 5 years. Fortunately he thought that she had a chance of bonding with me because of who she was and because of her age. Her brother, who is an adult now has never been able to bond with anyone. He has young children and he says that he loves them. I'm sure he does but when we spend time with them he is distant and not at all attuned to their needs.
Did your husband have frequent positive contact with her during those first years? Has she bonded with both of you? Is she bonded with her mother? and/or her father?
Bonding put simply is the ability to love and be loved. That is far too simple of an answer. I suggest that you talk with a child psychiatrist to determine if you even have a chance of influencing her. If she doesn't care what you think or trust that you will help her you will have a very rough time and it is most likely that she won't succeed in school.
You've said that school is not important to her except for the social aspect. If education is not important to her and you don't have the authority to consistently set boundaries and consequences how is she to learn to take school seriously?
Without having her in your custody the majority of the time I think that all you can do for her is to love her in the way that feels like love to her. If love means being allowed to do whatever she wants then she will fight you not realizing that she is pushing your love away. The job is to find a way to love her that is beneficial for her. That is part of the bonding process that usually takes place with someone during the first 5 years of life. If she's bonded then she most likely can bond with other's later in life.
The advice to love her by being firm but not angry is the most important start. Accept her as she is. You shouldn't accept her behavior but continuously remind yourself and her that you love the essence of who she is.
You do have authority over her when she is at your house. She needs you to have boundaries and consequences when she's with you. That is just the beginning. She needs your love, your boundaries and your consequences so that she can both trust you and feel that she is a good person who deserves a good education. When her mother lets her do whatever she wants, doesn't encourage her to study, or teach the values you're trying to instill, your daughter feels that she's not good enough to have anything any better. In fact she probably doesn't even know that there is anything any better. And most likely that she's even capable of success.
Sounds like you arranged the community college thing. If so, or if you are allowed to, I also agree that the best thing that you can do is take her to the school (arrange it with the school first) and have them tell her the consequences of not keeping up with school until spring when she can start there. After all it is the school's consequences. If you want to add any additional consequences you can. Since I don't know how much time she is with you I can make no suggestions along that line.
I strongly urge you to get some professional help with your step-daughter. Now that she is 16, tough love won't fix it. You only have 2 more years to be an authority in her life.
I suspect an education is not important to her. At least not important enough for her to work on getting it. She may even believe that she can't do the work and so doesn't try. Arranging for the community college education is a great idea. But if she doesn't qualify there is nothing that you can do for her as far as school is concerned.
If she doesn't get into the community college I'd back off the emphasis on pushing her to succeed in school and focus on teaching her self respect and love for herself. That is a very difficult thing to do. You will need professional help.
My daughter and I were in counseling from the time she was in the first grade until she became an adult. Fortunately I was able to instill her with a love for learning and reading and she has succeeded in that. Unfortunately she has a very difficult time with relationships. Our relationship with each other was and is difficult. We would not have lasted without professional help.
I am also a retired police officer who worked in the Juvenile Section for years. I taught high school before that. Kids at 16 have a mind of their own even when they've been raised in a caring consistent way. Building a trusting, loving relationship is a top priority for you and your daughter.
I am glad that she wants to go to the cc and you should emphasise the success that she can achieve. If she's able to make the transition and learn to study you have something positive to build on. If she isn't able to do that, just love her in as positive a way that you can manage. Have rules and consequences that you and your husband can enforce while she's with you and work on consistently enforcing them. As painful as it is, allow her to suffer the consequences of her actions. The consequences now are for the most part not as serious as the ones she'll face as an adult.
Keep in mind that some teenagers do better with a GED and occupational training. And they may flounder during their teen years but discover as they mature that they do want something more out of life. There is no set formula for raising her to be a successful adult. I do believe that having and feeling unconditional love is necessary.
Focus on loving her as she is, school or no school, and provide boundaries and consequences in your home, Tell her often that you love her and, when you can, that you're proud of her. Give a whole lot more praise than criticism. When you can discipline without anger you are showing her love. When she is with you provide activities in which she can succeed so that you can praise her more. Work on finding a way for her to have a social life outside of school. Learn about activities available in your community and encourage her to go. Get to know her friends so that they can go together.
I hope I'm not stepping out of line here but there are some very serious consequences for her behavior right now. Pregnancy and STDs. Work with her to get protection from those. I know several girls who either wanted to finish high school or continue on to college but they've had to postpone college until their children are older. They have a very limited income and at first were just children themselves. They, their parents, and their babies are having a really rough time.
I feel for you and the frustration you must have. And the anger because her birth mother is not providing her with what she needs. We do want what's best for our children. I have been so blunt because it is frequently not possible to provide the best. If you are unaware of the skills and difficulties you have when your child is 16 and not in your sole custody you are setting yourself up for a life of frustation and anger which will make having and showing unconditional love more difficult.
Unconditional love is the most important need that your 16 yo has. If you can provide that you will influence her life even tho you might not be able to see that she gets an education or do other things that seem essential to you. Boundaries and consistent consequences in a non-judgemental way are a part of that love. You can only provide those when she's in your home.
I empathize with you and so have written in a blunt manner. I wish you well! I hope that you can set boundaries for yourself so that you will only take on what you can handle and remain healthy and in a healthy relationship for yourself.