There are many strategies you can try to motivate your daughter to make better choices. Make sure that the consequence that is issued can be followed through. That could be why the grounding is not working. Ask yourself: What does grounding mean? What is the message that I want my daughter to understand? What can I do to help my daughter? (Just by reaching out for help is a method to help her!)
There may also be some other factors to consider. What has changed in the environment? Does she have good friends? Does she feel she has time? Does she understand what is the instruction or expectation? Sitting down and talking with your daughter about her choices can also establish how you want to proceed. As a parent it is very frustrating when kids do not follow the rules or expectations set by the school or the expectations in the home. Talk with her teacher and/or school counselor about the change in her behavior.
You can be the encouragement and support that she needs to overcome this terrible choice that she is making for herself. It is also a good time to really understand what is going on. You mention that she simply forgot to hand in the homework or that she did not want to do the paper. She is making excuses. You can start teaching her some self discipline and independent skills. Think of teaching your daughter, rather than punishing her. You may have to set up some goals and expectations. You can do this together or leave it up to her to come up with a plan of how she will change this choice. This exercise alone could be a consequence. Having her put pen to paper to write this out, could be like her not wanting to complete the other work. This is a good teaching tool to build from. Explain to her that there are many things that you will have to do in your lifetime that you will not like or want to do. You need to fill out paperwork, fill out applications for employment,etc. Think of all the tedious paperwork that you do not like to do and use that as an example. Once she is able to follow the rules of the task given to her, she may earn her privileges. Maybe only for a little time, may she have access to her privileges (free time in the home, hanging out with friends, video games, computer, television, etc). You can also set up some chores for her to finish and accomplish. By giving her responsibility, it will teach her that she will have to juggle many different daily activities--chores, going to school, doing homework, etc, are all apart of her daily routine right now. She will then transition later in life--with job, family, bills, chores, etc.
Remember she also needs to feel like you notice all the good choices that she does make. If she is attending school, praise her. If she is able to talk to you, praise her. If she has good friendships, praise her.
If you or your daughter need further assistance, you can call the Boys Town National Hotline at 1-800-448-3000. We have counselors available 24/7 and we talk with kids and parents about various issues concerning them. Take care and consider giving us a call.
It will be helpful to know her grade level. One definite step you should take is to have a meeting with your child's teacher, your child and you. I suspect that there is a lot going on with your daughter that your message does not address. I should not speculate on the wide range of issues that may be a cause for this problem; however, I do believe the web resource cited below may be helpful.