Yes you can. Most schools will accept your highest score on either test, regardless of how many times you took it.
Some experts, however suggest that after 3-5 times, your score is not likely to increase significantly. Taking the PSAT/NMSQT to prepare for the SAT and the PACT to prepare for the ACT are also helpful as they provide a predicted score range and may show you areas of personal strengths and weaknesses.
Start at least during your junior year. This gives you time to improve on any weak areas and give the schools you are interested in the opportunity to start sending you materials.
ACT statistics show that 77% of students' scores stayed the same or went up after re-testing, so there is little reason not to retake the test.
Statistics for the SAT were not found.
How long do the tests take?
Both the ACT and SAT include about three hours of testing, although with a break in the middle and time between sections, you will likely be at the testing center a little over four hours.
What can I do to prepare for the tests?
Get plenty of rest the night before
Dress comfortably and in layers – in case the room is hot or cold
Arrive 15 minutes early but not more (testing centers are rarely open earlier)
Know the instructions and format for each section
Answer the easy questions first
Check over your work if time allows
Be careful on the answer sheet – mark answer ovals completely, erase changes completely
Read information on individual tests about guessing
On reading sections of both tests, read the questions before reading the passage. You will then know what to look for in the passage
Absolute words such as "always", "never", and "all of the following except", demand an absolute answer
What should I take to the test?
Take a watch, digital if possible. Set at 12:00 at the start of all sections (makes it easier to tell when time is up)
Take a calculator – most four-function, scientific or graphing calculators will work. Be familiar with how to use it
Take an official photo ID and your test admission information/slip