You definitely don't need to be naturally intelligent. What you do need to be is smart in terms of how you study - identifying the topics you struggle with, figuring out the best ways to answer questions, and improving exam technique in general. You'll also want to study regularly throughout the year - review what you covered in class when you get home, and review again each week/fortnight/month to make sure you're still familiar with the content. Don't just study for tests, study to retain information and understanding throughout the year. Continually ask your teachers for advice on where you can improve, and spend a fair amount of time practicing exam technique - practicing essays, or questions for certain subjects. For practical subjects like Music, make sure you spend time practicing. You don't have to be smart to get good grades, you just have to study smart.