A University of Pennsylvania study found that self-discipline is a much better indicator of academic achievement than scores on intelligence tests. This is because intelligence is not particularly helpful if a student does not have the self-discipline to apply it; if you're capable of doing a project of an extremely high caliber, but simply decide that it's not to your liking and choose not to do it or to do it half-heartedly, your academic achievement will suffer. This is a major difference between intelligence testing scores and academic achievement scores or grades.
Academic achievement has a clear result: if you get good grades, you are more likely to get into better university programs and/or get scholarships to help pay for those programs. Intelligence testing, on the other hand, does not have a clear result. Consequently, some students are more motivated to achieve academically than they are to score high on intelligence tests, as college and scholarship application officers do not refer to I.Q. tests in making their decisions.
Academic achievement occurs over time. It is the aggregate of hundreds of tests, quizzes and assignments, while intelligence testing is the product of just a few tests held over a day or so. Thus, academic achievement testing is an aggregate of a larger number of tests. This means that if the student has a bad day during one test, it will affect his aggregate only a little, but if a student has a bad day on the day of an intelligence test, he may be saddled with a low score unless he retakes the test.
Academic achievement is measured by teaching and testing in subjects such as English, history, mathematics, science and other academic subjects. Intelligence testing examines more abstract concepts, such as your ability to connect ideas, conceptualize shapes and generally think in a critical, abstract manner. This means you can't study for an intelligence test like you can for an academic test. The two tests measure two different things, and while there is some correlation, it is the same correlation that apples and oranges share: they are both fruits, but the similarities end there.