This. I'm still in high school (going into senior year) and my parents are the same. If I do poorly on a test, or assignment, or anything school related, I can guarantee I won't be allowed to ski that weekend. My parents are extremely strict about grades, yet I get nothing in return for having a 3.9 GPA. This is not supposed to be some sort of claim, because I hate working hard in school. It's boring, for the most part, but I understand why my parents want me to strive for high academic achievements.
Although I do have sympathy for you, you should have tried a little bit harder in school. If you legitimately tried as hard as possible and still got a C+, you should explain that to your parents, and I bet everything in the world that they will let you go. The problem is probably not that you're "stupid" because I'm sure you're not, but merely the laziness and procrastination that's rampant in high school.
So good luck, and I'm extremely jealous of everyone who gets to go to Windells. I'll see you guys this summer in the public park.