HELP?!

Does anybody have any suggestions on how to motivate a 15 year old, who has been testing at College levels since 6th grade, to get grades above C? We are at our wit's end. We have bribed, punished, begged, punished some more and I am just about to sit down and cry. This mom thing is hard.

Only 1 week and 1 more day until Disneyland! I can not wait. I need a vacation. Too bad I have to take DH & the Ks. :)