I am not in high school anymore, but I just started my undergrad and had a similar situation where I got a C on an exam last semester and a C on an exam this semester, even though I understood the subject matter and could tutor other students in it.
What I did to remedy the problem:
- went over the mistakes on my own and then discussed any ambiguities in the grading with the professor. In both cases, I was able to negotiate my grade up by five points via this, which moved my two exams into the B range.
- asked the professor for specific advice on doing well in the future. The interesting thing was that they gave me different advice.
The first prof, in mechanics, told me to ask the instructor questions about any parts of the exam where you feel like you're making an assumption that isn't stated in the problem. This helped me tremendously later in mechanics, because I stopped overcomplicating exam problems. I ended up getting an A on the final and in the class.
The second prof, in E&M, said that I was concentrating on challenging problems when preparing for an exam, which showed that I could understand E&M conceptually, but also meant that I wasn't spending enough time cracking the really easy questions. So I'm going to switch my study strategy to solving more 1-point and fewer challenge problems.
Your teacher may be writing his or her exam in a way that you're not used to studying for. I would ask him for a good approach to preparing.