"Grades don't measure how smart a person is."
In my opinion, school isn't what makes people "smart". Most of the values, skills, and lessons that we pick up as we grow are taught outside of the classroom. So what if you got a low score on the SAT!? You're not stupid. You just either don't care or you're a poor test taker. Yes, grades are important in pursuing an higher education and a job, hopefully resulting in a successful career. But face it, at the end of the day we're going to need those outside-the-classroom lessons before we need to know equations and theories.