One clarification. The PSAT/NMSQT (used to decide on National Merit) can be taken during sophmore or junior year.
Here is a link that should answer most of your questions about the National Merit program: http://www.nationalmerit.org/faq_competition.php
please note if you go to high school for 4 years, it is your junior year test that will be considered. But as mentioned previously, many students take the PSAT as sophomores as sort of a litmus test to see how they do and to just get practice in taking the test. And if you graduate a year early, the sophomore year test can be used to qualify for the NM program.
Getting good grades and having decent test scores are helpful in finding scholarship dollars to help pay for the costs of attending an MT program since talent money is often limited. So encourage attention to academics during high school 
Yes my D took PSAT this past fall. She did well just not great in math…of course! The SAT is actually changing for the class of 2017 which is my D . We just had a meeting about it .
My daughter was really closed minded about taking the ACT because of the science component. Not sure why as she took Honors Physics, Chemistry, and Bio, but definitely had a mental block. She thinks of herself as more of a English/history
Accidentally posted before finished…thinks of herself as more of a English/History kid than Math/Science. She took SAT twice and did well, but when took ACT just for comparison- killed it! Definitely try both, especially with upcoming changes.
I have 2 daughters. One did much better on SAT, and one much better on ACT! So yes I’d strongly second or third suggestions to take both tests, even if only once, to see how it goes 
Both D’s were able to do well on ACT science the second time taking the test. They took a few practice tests and once they got the hang of it, didn’t really need to study much. One D did significantly better in math on the ACT. A score of 35 vs. a 650. The other D essentially the same on both tests.
Although the ACT has a Science component, it is not really testing the student’s Science knowledge. It is more like a speed test - how fast can you glean and process information as presented in different formats. For folks with reading and processing issues this test may require quite a bit of practice or testing-taking strategy tutoring to do well. Even a student with extensive Science background may come up against a wall when it comes to the speed needed to think through the questions - I think it averages out to around a minute per question. That’s awfully quick if you’re feeling stressed.
For MT/Acting students/families looking for “less selective” schools academically, is it because the student has a “learning issue” (dyslexia, processing speed, ADHD, executive function, etc.)? MANY people in creative fields have to deal with these brain “glitches” that make success in an academic environment harder to attain. If this is so for you/your child, you might also want to look at schools that give strong support to students with those issues. Not all schools are equal in this regard. Schools that promote higher education for underserved populations and “first generation” students tend to be a little better in this area. I know I’m a broken record about Wright State, but it is one of only three schools in the state of Ohio that has a dedicated Disabilities Services department to aid students with “documented disabilities”. The full program costs extra money, but there are also many free services to help students be successful that do not require documentation of disability. There may be similar programs at schools in other states. Some students may also do well at a community college that is focused on academic success for non-traditional students. This is a terrific way to get gen-eds on the cheap in a supportive environment and still be able to get MT/Acting experience and training outside of school, then perhaps transfer to a larger/more competitive setting when the student has matured a bit.
By its very nature, the CC forum population tends to be skewed towards overly involved/interested/educated/obsessive people looking to control every aspect of their, or their children’s climb to “happily-ever-after” (Yes, I’m guilty of this), but working hard at a “craft” and getting a decent college education can come in many different forms that may look “heretical” to members of extended family, neighbors, teachers, and the greater community. It’s OK, if your path doesn’t look like “everybody else’s”. Breathe.
If you can take both practice tests, or the PLAN and the PSAT, that might be a good indicator. I didn’t do very well on the science section of the PLAN, and I hated taking it (there is way less time), so when it was time to take the real tests I stuck with the SAT.