Take a second and go back to high school, sometime probably middle-to-late in your junior year, possibly even early senior year. Right about when you're trying to figure out what to do with your life after the year or so you have left of sheltering under this comfortable wing that you've built your entire life.
At this point in your life, my guess is you were probably thinking about the ACT or SAT, freaking out about having to pass it at a certain level to get into the college of your dreams. I wasn't any different when I was at that level either. What's interesting about these tests, to me, is something that was pointed out in chapter nine of the Lilienfeld book, about whether or not these tests actually predict grades (or, as I interpreted it, vice versa). These tests, as stated by the book, are "either to test overall competence in a specific domain or to predict academic success". So, ideally, these tests that we all stress about, and ultimately that little number we receive at the end of it all, is supposed to be an ultimate indicator of how good a student we are and can be. The problem with that is this: While these numbers may somewhat relate to our overall abilities as a student, they are not very accurate.
Here's a real-life example from my own experience taking the ACTs. In my high school, I with a 3.7 GPA and mostly As. I rarely did any hard studying, mostly getting by pretty easily with just doing the homework and accepting a few wrong answers here and there. When I took the ACT I came out with a 32 (out of a potential 36), which is a pretty high score for this particular test. One of my best friends, however, who was top of our class with a 4.2 GPA, studied hard every night, talked with teachers on her own time if she needed help with something, you know the drill. However, her ACT score topped at a 28.
Now, arguably if the ACTs, or conceivably any other admission test, was going to work as it's supposed to then my friend, through all her hard work, should have received, if not a 36, at the very least a higher score than I did. I would say that she could have arguably received a 36, with her GPA and grades up as high as they were. Clearly this isn't happening. Even in the graphs presented in our textbook, while there is an upward correlation between GPA and the SAT score, in this case, it is very general and can only be seen when looking a large range of SAT scores (700 to 2300 vs. cutting that in half to a 1500 to 2300 range).
What does this say? Well, that in a very, very generalized way the standardized tests can predict grades, but only in that if you have a significantly higher test grade than someone else that your GPA is likely higher as well. Not a really huge correlation, I would say. Better tests are being formed, as seen here, and some more interesting facts about it all here. But in the meantime, don't judge your IQ, or your ability to get a good grade, just by your test scores.