School is for learning and is about improving academic skills/intelligence. The ACT and SAT are centered around things that you learn in school, and things colleges look for (grades). It's based on the essentials of all the majors. Math, reading, and writing are the basics you need in life, other skills also need these skill to be successive. It's the building blocks.
I am in high school and recently took the ACT , although I got a satisfying score. Throughout most of the time we were preparing we learned how to not get a bad answer and mostly guess on the correct answer. This is not because my school does not care but because we thought the ACT is more of testing skills rather than actual learning and putting lessons to the test. We mostly got scores ranging from 19-22. Many of us had different learning habits and different courses than the other (honors and AP for example) yet they still scored almost the same as those who are taking those courses
ACT and SAT tests are not effective towards accurately assessing a student's intelligence. When a high school student takes one of these tests, they are only measuring some facets of their intelligence. If a student is particularly adept at something not tested on these tests, like mechanical skills, they do poorly on the tests, even though they are intelligent in other things.
I do not believe that standardized tests such as the SAT and AST are effective towards accurately assessing a students intelligence due to the primary fact that some students are just not good at test taking. Many students suffer from test anxiety and no matter how well prepared they are or how intelligence they are they will not do well on the test.