I have a 16 year old who is similar. She sometimes does the work, sometimes turns it in. The teachers give out monthly progress reports via email and when there are 0's it really effects the overall grade. Having the report so the child can see what it does to the grade is a good tool for getting them to see the consequences. We have taken away social priveledges, taken away computer time which is also a social thing, and implemented mandatory homework time after school in view of me (because I am home usually) so I can monitor. (otherwise she would just be in her room with the door shut and who knows what she was really doing)
Reminding her to put everything in her backpack has helped, but ultimately the child just has to GET IT at some point. The removal of social things has been somewhat helpful because that is something I think most high school age kids desire. I am also in contact with all her teachers weekly at least to keep up to date on her progress.
She has an agenda book in which to record assignments, exams, projects, etc. This is good, as long as they remeber to use it, which I remind all my kids to do often.
reward for good grades has been successful some also. Five dollars for an A, 3 for a B, we don't reward anythign lower. Money can be a good motivator when they see that it can help them become more able to do things with friends.
we live in Stuttgart, Germany so our kids are at DoDDS schools. I find a lot of parent interaction is necessary because the system overseas hasn't been wonderful in our experience.
try to be patient, pray, and don't get too down on yourself when your kids don't do so well inschool. I am guilty of beating myself up over their lack of success, but have learned that it isn't the parents fault most of the time. We do our best!! The fact that you are reaching out for help is good in and of itself!