The ACT measures your skills and knowledge in English, math, science and reading. A good time to take the exam is the spring semester of your junior year. Advantages to testing in your junior year are:
You've probably taken the courses that prepare you for the test.
You'll have your test scores and other information in time to plan your senior year. For example, you may decide to take an additional class in a subject in which your test score was low.
Colleges will know of your interests and have your scores in time to contact you during the summer before your senior year. Many colleges send students information about admission, scholarships, and special programs at this time.
You'll have more information about yourself and the schools you're interested in before campus visits, making your visits more focused.
You'll have plenty of time to retake the test before applying to college if you feel your scores don't accurately reflect your ability. ACT research shows that of the students who took the ACT a second time, 55 percent earned a higher composite score, 23 percent earned a lower score and 22 percent stayed the same.
The decision to take the ACT more than once depends on your own situation. If you're happy with your scores, you don't need to take it again. But if you think you'll do better a second time because you'll know what to expect, you've taken additional classes, or you've received tutoring, then you should take the ACT again.
Talk it over with your parents and counselor if you're uncertain.
Source: ACT's News You Can Use