I don't have kids. I'd try stern warnings (with reasonings why they shouldn't be doing what they're doing, just to make sure they know what they're doing is wrong, and why), then depravation of privilages, then work (be it a job I don't feel like doing at the moment, or just push-ups), and if that doesn't work, corporal punishment. Failure to get the idea with one concept would force me to move onto the next concept. Actually, there are some things I wouldn't use corporal punishment for (failing grade is one of them), I'd just do more severe punishment of the previous steps.
I don't exactly have room to tell my kids (whenever they get here) to get all A's, but if they brought home an F, I'd hold them to the same standards I held myself to in school. If the F was for a single minor assignment, I don't really care. If it's for the class, they'd better get working, and I'll do my best to help them, even if it means removing distracting privilages.