High-schools throughout the country have varying grading and instructional standards, high-school G.P.A. by itself is not an accurate predictor of college performance. The SAT serves as a standard by which colleges and universities can measure a student's reasoning abilities to predict how well he or she might do at their college.
The SAT does not test how much you know. Rather, it tests your math and verbal reasoning abilities, and compares it to the abilities of students from other high-schools. Each of the two sections on the SAT is scored from 200(lowest) to 800(highest), for a maximum of 1600.
