The decision to retake the SAT and/or the ACT is a significant one, and you must consider several questions in order to reach the conclusion that is correct for you. Your decision should be based on several factors, including, but not limited to: how many times you have already completed the test, at what point in your high school career you last took the test; and what you actually stand to gain by sitting for the test again. Consider the below questions in order to make a sound decision:
Is a school requiring a certain score from you?
Does your dream university demand a given mark for admission? If so, you must work to achieve that score or else you may end up submitting an application that does not compare favorably to those of other students vying for the same spots in the first-year class. You must be realistic here: If you scored a 15 on the ACT the first time, it would not be wise to expect a 35 on the second attempt. However, if you are hovering just below the cut-off for scholarship funds (say, you are only one or two ACT points shy), then you should strongly consider retesting.
Do you definitely have the time to devote to another round of test preparation?
Be honest with yourself. There is no sense in retesting unless you are going to be serious about the effort you put forth. Do not expect to improve your SAT composite score immensely just because you completed one practice test the week before the exam. Beating the odds for score improvement takes dedication! If you have soccer practice, student council, debate team, and play rehearsals that are demanding all of your time this semester, you may need to seriously prioritize your study sessions, or seriously contemplate not retesting.
What will make this time around different than the last time?
This question is undoubtedly a broad one, but it is very important to consider when making your decision. Was there a family emergency greatly distracting you during your original test day? Have you now taken considerably more mathematics courses and therefore understand the trigonometry better on the ACT? Have you been avidly reading every day for the past eight months to ensure adequate preparation for the critical reading portion of the SAT? Did you have considerably less time to devote to studying the first time around and have a much clearer calendar now? What will you truly gain by improving your score? It is imperative to discover what will set this time apart from the first. If you can't generate a strong list of reasons, reconsider whether or not retesting will actually be beneficial.
Retaking either of these tests could be a significant game changer, but it could also be a significant waste of time if not approached correctly. Do not take this decision lightly, but don't be afraid to contemplate it either. Take the time to thoroughly evaluate the situation and determine what possible next steps you need to take in order to secure your academic future. Think carefully and good luck!