How can I keep up with my work and get good grades throughout High School?

I am in 10th grade and I go to a private High School and I don't mean to brag, but I am very intelligent. I am one of the smartest kids in my class and I know that I can do very well in school, but I have a hard time keeping up on my homework and studying for tests, or exams, or things like that. I recently took the PSAT and I got an overall score of 164. Also worth noting is the fact that I score higher on "critical reading" than 95% of kids my age, higher on math than 63% of kids my age, and higher on "writing skills" than 77% of kids my age. I had the highest overall score in the class and take into consideration that I go to a private school that has much more "intellectual" students than the average public school. 98% of kids in my school move on to higher learning after high school. So, the problem here is: I am very intelligent and I can do well on such exams as the ACT, PSAT, or SAT, but I can't seem to keep up with the extreme, massive, copious amount of homework that is assigned in my classes. It is an absolute shame to see other students, that I know aren't as bright as I am, doing better than me. My GPA is not the greatest and I just can't keep up with all the work. For example, my Algebra class is sickening. The amount of work, coupled with the overbearing extremity of the material involved, is unbelievable. I really don't know what to do. I am under the impression that colleges look at GPA above all else, and mine is much lower than my capability. Please help me.......It would be horrible to let my opportunities go to waste. Once again, I do not mean to brag or boast, I am just trying to tell you about my personal feelings of myself in a matter of intelligence. Thank you for your time and I hope you can help me.