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What is more important? Grades of test scores?
A: GPA--combined with the quantity and level of your high school courses--is of primary importance (a strong GPA, for instance, on an academic program that lacks honors level courses or does not include courses well beyond those needed for basic UC eligibility, will not likely result in admission). That being said, test scores are still an important element.
First and foremost, you should take the most challenging courses you are able to handle. This is also the best way to prepare for your standardized tests. It is also a good idea to be prepared to take the tests more than once--in other words, don't wait until your senior year. That way, if you do not perform your best on some tests, you have a chance retake them in the fall of your senior year.
We also find academic strength in AP exams with scores of 3 and above (and IB exams with scores of 5 and above).
It should be noted, however, that because UCLA does not use a mathematical formula when evaluating applicants, there is no fixed numerical weight attached to either GPA or test scores.