We all know how the grading system works. A is the best score you can get (and the only acceptable score for the perfectionists among us). Then comes B, which is also generally considered positive. By C, you’re in dangerous territory. D is even worse. And F, well, F is the equivalent of wearing a dunce cap. But where’s the E grade? Why do we skip over this letter? Turns out, we didn’t always exclude the letter E from our grading system. And it l…