In fact, nothing could be further from the truth. The reality is simple: these tests present material in their own "language" - if you want to get high scores, you need to learn how to speak it. Most students and parents assume that, because they have high grades in math and English, these grades will translate into high test scores. They're shocked when this isn't the case, but they shouldn't be - the SAT is far from a simple math test. It combines esoteric wording, material from multiple mathematical disciplines, logical reasoning, and time pressure to create an entirely new testing experience.
If you want to get good SAT and ACT scores, you need to prepare for the SAT and ACT - NOT for the isolated material that's tested by these exams.
In other words: it doesn't matter how good your math, vocabulary, reading, and grammar skills are if you've never applied them to the SAT / ACT.
A good analogy: a man spends years and years studying Russian vocabulary words in school. Every day, he learns a few more, and every week he's tested on their meaning. He's a straight A+ Russian student. Then, one day, he's flown to Russia and asked to speak to a bunch of Russians in their native tongue...and what happens? He'll have no idea how to do it. Sure, he might have all the vocab words down, but he doesn't have the grammar, the ear for the language, the rapidity of comprehension, the idioms...the list goes on. And so, even though this student has the material mastered, he still can't speak Russian.
Does this mean the student is "bad at Russian?" No. Does this mean the student "just can't do Russian?" No. It means that he has never spoken Russian before, so OF COURSE he isn't good at speaking it AT THAT MOMENT!
If you want high SAT / ACT scores, you can't just know the material - you need to be good at taking these tests. And these tests are unlike anything that you're probably used to. While tutoring and self-study programs are both good options, classroom courses are the absolute worst way to prepare for the SAT, the ACT, and any other standardized test.
Classroom SAT and ACT courses replicate the EXACT errors in instruction that prevent students from getting good test scores in the first place!