Respect and Grades

Mom get a handle on this NOW! She needs severe consequences for this behavior. Spell it out for her what the expectations are and follow through with defined consequences. (take the important stuff away!!!) You need to take her in and have her apologize to her teacher. This is disrespect of authority. It starts with the teacher, and most likely she does it to you. This looks very ugly as a 16 yr old let alone as an adult. NOTHING happens after school until that homework is done!!! NO TV, COMPUTER, PHONE, FRIENDS until the work is done!!! Come on mom, you are the only mom she will ever have and it's time for you to step it up and do your job.
YOU can do it.

our son has been having a problem with homework (and grades)... so we hired a tutor, now she handles a lot of that for us, he really likes the one on one attention that he gets, and it takes a lot of the pressure off of me telling him he has to do his home work, she makes it "fun" for him and he has gained confidence. sometimes the tutor can get to the root of the problem that your child is having at school and her her to get through it.

but we also speak with our child so that he is aware of our expectations, and knows what is acceptable and what is not, and what the consequences will be if he does not do what is expected of him.

Have you had her tested for a learning disability? If she is frustrated, she might be acted out rather than letting anyone know she is having problems.

Check out the Parenting with Love and Logic books (you can check out their website, too--they answer this exact question). I think you'll find them very helpful. I think they have the books in the library too. http://www.loveandlogic.com/

What is she doing that is disrespectful? Have you asked her about it? If she used to like school and learning, there may be a specific problem with Tiffany's relationship with this particular teacher or her teaching style. (no that is not automatically putting the blame on the teacher - but some teachers are great and some are not and you may need to be involved in the process so that Tiffany can truly enjoy school) Punishing or rewarding her for grades will not encourage her love of learning. This is something she needs to internalize.

Good luck, my step son is now 15 and that's about when poor grades and not caring started.
Try a reward system with her to see if that gets her going. Almost like reverse psychology. Worked for a little while with my ss, he just flat out didn't care if he passed or failed until the very last possible minute and then he would charm his teachers somehow and he would pass.
We tried consequences - took EVERYTHING out of his room, not allowed to do anything but basically follow us around. Well he figured out it got to us more than him.
Like i said start with figuring out some sort of punishment and/or rewaRd that you haven't done to see if that gets her going.
GOOD LUCK! I've got 3 more to go so I'm hoping by the time I get to the 4th I'll have it figured out!! :)