I strongly recommend writing the test more than once (ideally 3 times), to maximize your chance of getting your best score. Think of it like this:
You might give up after 2 attempts, because your score dropped, but really your average score is closer to your first test. The one you actually want to submit is your third test, but you need to know where that score falls on your personal scale before you decide whether to write again or not.
In my experience, students’ test scores vary around their mean score. So let’s say your mean ACT score is a 27. The first time you write, you get a 24. Then a 26, followed by a 28, then a 27. You need to know what your mean score is, otherwise you won’t know what your highest possible score could be. Each test is different, so one test could suit you more than others for a totally random reason. If you only write once and get a 28, that could be your highest possible score, but statistical variation in the test could get you a 30 the next time you write.
Also, you’re more likely to be motivated if you’re only 2 points away from the score you need to get into your dream school. The other thing about only writing once is that students who are doing this are usually doing it on their last possible test attempt, which means added pressure – if you write more than once, you always know you have another chance, which really lowers stress levels in a timed test!
One of my students, Allen, wrote the SAT twice, with only a month in between. He didn’t do any work in between because he was studying for end of year exams, but his total score improved by 100 points. This isn’t a huge difference, but it’s worth giving up a Saturday morning for! There are two possible reasons for this: either one particular test happened to suit Allen more than the other, or studying for his school exams helped his score as well.
Some people think that admissions officers are expecting you do get a great score first time around. This isn’t true – achievement takes perseverance, and I’ve heard admissions people say that they may even look positively on multiple test attempts if the student improved. This might not be true in all situations, but it certainly is in some!