The longer you spend in high school, the less your GPA can move, so as you progress through the years, shift your focus to the part of the admissions index that can shift: your test scores.
Your GPA is almost unmovable as a Senior:
Let’s look at what would happen to a senior with a 4.0 GPA who got a B instead of an A in one class in his 8th semester: his GPA would move from 4.0 to about 3.98. Similarly a 6th semester student who had a 4.0 who got a B instead of an A in one class would experience a decline in his GPA of 0.024 points–an inconsequential shift. In fact, if we run the numbers, a senior with a 4.0 who got a full semester of Bs would only see his GPA move by 0.14 points.
So what does this mean for me?
You should prioritize test prep once you are past your past your 5th semester. Your standardized test scores are, unlike your GPA, always able to be increased, and actually improve in the later years of high school. Consider this: you only have a fixed amount of time every night, so should you spend most of your time on homework to keep your 4.0 at 4.0 (0% return on time spent), or should you spend most of your time on test prep to move your 30 to a 32 (7% return on time spent)? Even if you spend most of your time on test prep, and you get a B instead of an A in a class, we can see that your GPA will only decline by about 0.02 points (a .005% decrease) in exchange for a 2 point gain on the ACT (a 7% increase).
Rethink your Junior and Senior Years:
This is especially important for current seniors: don’t let school work dictate your schedule. Your GPA is all but immobile at this point, but your test scores are flexible and easily shifted with focused prep. You can drastically improve your chances of admissions to your dream school with an extra two or three points on the ACT, but you cannot drastically shift your GPA at this stage with two or three extra A’s.