How I feel for you. Well pardin my French but you are going to have to get real, real shitty with her. I mean at this point you have only taken away priveleges you need to take away more than that. So you go in her room and you take everything away, her blow dryer, her favorite pair of shoes, and don't forget her clothes, etc.
The only thing she should have in her room is her bed, a pillow, and the sheets and blankets on her bed. You see we as parents only have to provide her basic needs...clothes on her back, food in her belly, a roof over her head, thats it. Let her know that until she can do her school work you will pick out her clothes, you will pick out her shoes, she will eat the food that is in front of her and the drink in her cup (water of course), let her know that if she can't handle doing her school work than she can't handle the finer things in life and the responsibility that goes with it. I'm not kidding either, you see she thinks shes got you over a barrel, this is the battle of the wills. She thinks there is nothing you can do to make her do her school, boy is she wrong...you can make her life a living hell. Let her know when she starts coming to her senses she can start having her things back.
You see I went through this with my son (16 yrs old, only it was in his Freshman year of high school, and we took eveything away and I mean everything. He is now to the point that he has 2 hours of playing Xbox or computer as long as he has no homework to do. If he does homework has to be completed first. My kids go to the Independence school district and on there they have what is called power school, where we can log in and see what the teacher has assinged for them to do and if they have turned in previously assigned work. He is salso able to go hang out with his friends as long as all assignments have no 0's.
I'm telling you hit it hard now with her, make it her worst nightmare, make her realize that doing homework is the best option she has. Let her start from scratch with the things she has. Don't let her out of your sight, don't allow her to go in her room, make her stay with you guys as a family to see what she is doing now and what she could be doing. Don't let her answer the phone, don't let her talk to friends, let her know that if you have to you will take a leave of absence from work and homeschool her.
I have been down the same road you are on right now, Thank God I am on this side of it finally. Well I think you get the point to what I'm saying, so I'll hush now. Anyway goodluck W.