Most students take the Math SAT subject tests at the wrong time. And, it’s not their fault.
Here’s the standard advice from their Math teachers at school: when you finish Algebra 2 and Trigonometry in school, then take the Math Level 1 and/or 2 SAT subject tests.
On the surface, that’s good advice. The Math Level 1 and 2 SAT subject tests cover material from Algebra 1, Geometry, Algebra 2, and Trigonometry. Thus, the rationale provided by teachers to their students makes sense: take the Math SAT subject tests when you’ve finished learning all of the necessary subjects in school, otherwise you will just forget more of the material if you take it later.
What’s wrong with that standard advice?
By that same logic, if a student finishes Algebra 2/Trigonometry at the end of their sophomore year, they should also take the SAT and/or ACT at the same time and be done with the tests, because both the SAT and ACT also only cover up to Algebra 2/Trigonometry. So why don’t most 
students who have just finished Algebra 2/Trigonometry take the SAT or ACT at that time? Because they haven’t yet studied for the test. And, for the Math section specifically, they haven’t yet gone back to review and, typically, re-learn the Algebra 1, Geometry, Algebra 2, and Trigonometry that they have forgotten. As this video shows, the only way to reach one’s potential is to pour skill and knowledge in faster than it pours out; that can only be done through concentrated study and review.
If a student does not plan to prepare for the Math Level 1 or 2 SAT subject tests, then, yes, he or she should take the tests right after finishing with Algebra 2/Trigonometry. But, with a minimum of 24% of test takers currently achieving a perfect score on the Math Level 2 exam (as detailed here), almost all students who want to perform well will need to study for at least the Math Level 2 exam.
So, when should students take the Math Level 1 and/or 2 SAT subject tests? For most students, the best time to do so is after they finish taking the 
SAT or ACT because these tests already force them to review previously learned (and forgotten) Math topics. If taking the SAT, most students need a fair amount of prep to succeed on the Math Level 1 SAT subject test. If taking the ACT, most students need little to no prep for the Math Level 1, because the ACT has more questions from Algebra 2/Trigonometry and is thus very similar in content to the Math Level 1 SAT subject test. In contrast, for the Math Level 2, almost all students need substantial prep for this exam (though SAT students usually need a bit more than ACT students). Thus, for most students, they will reach their potential on these exams if they prep for and take the tests in May and/or June of their junior year or August/October of their senior year.
The above is not theoretical: we rigorously track our students’ results, and a much higher percentage of them achieve their top score after they finish with the SAT or ACT. And most of our students score a perfect 800 on the Math Level 2. But, just like achieving one’s potential on anything, doing so on the Math Level 1 and 2 takes planning and preparation.
