from my 16 year old son:
Parents need to remember that just because their kid is failing math does not mean he will fail at life.
I hear a lot of angst from parents about grades. It makes us feel so powerless to see our offspring not working up to their potential! We try consequences, punishment, privilege removal, microscopic supervision, bribes, and lectures. Sometimes, these things seem to work, and their grades improve.
And sometimes, we only cement our teens' determination to have some control over their choices. Academics are not a battle we can truly win, because ultimately, the motivation for all REAL learning comes from within.
Step into your teen's shoes (combat boots?) for a moment, and imagine hearing this from your parents:
Honey, I have no doubt that you can successfully accomplish anything you set your mind to. Let me know if there's anything I can do to help, and good luck!
At the very least, this takes rebellion out of the equation.
If things get tough for my kid, I want him to see me as a trusted resource who is on his team and will help him figure out what to do, rather than being fearful of my reaction and hiding the situation from me.
Not only that, but there's so much more to success than grades. Skills such as honesty, responsible communication, self-awareness, curiosity, creativity, and compassion are at least as important for success in real life as academic discipline. Even if your kid is not a great student (and mine is not), he can still be a great person.