Here is complete guide to the NCAA: The NCAA Guide for the College Bound Student Athlete
If you are one course short of the 16 core-courses required by the NCAA to compete at the Division I or II level, you can make that course up within one year of your high school gradation. The NCAA requires athletes to obtain a 2.0 GPA or better in 16 core-courses. These courses may include subjects in English, math, science, social science, foreign language, philosophy, and comparative religion. To see a full list of the required courses you will need to take to be eligible at the NCAA DI or DII level go here: (link to What Classes Count for my NCAA Core Classes FAQ). Remember, a 2.0 GPA in your core classes is the minimum. The lower your GPA is in these classes, the higher your SAT or ACT score will need to be. The NCAA uses a sliding scale to determine the ratio between GPA and standardized test score.
If you have completed 15 of the 16 required courses during your 8 semesters of high school, you are allowed to make up the one additional course you missed. The course must be completed within one year of your high school graduation. You can take the course at a location other than your high school, and once it is completed you can enroll full-time in a college or university at any point after your final core-course requirement is satisfied.