I'm assuming we are talking about high school grades.
High school is a bit of a joke, really the only thing that good grades can do is get one into a fancier school. However, they tend to cost lots of money, so if one's plan is to goto PCC or PSU after graduating C's are just fine.
If it is a motivational issue, try maybe getting some co-enrolement at PCC. That was a program we had at Aloha for Juniors and Seniors, free college! Less class time, shorter terms!