Grades, in general, are about playing the system. Do your homework. Study for the test. After that, it doesn’t usually matter much. If you forget something important, it will probably be reviewed again later. This is not always true, but for many classes, a letter grade means almost nothing. It should represent what a student has learned, but in many cases it does not. Nor does it represent the student’s intelligence level, as many people assume. It represents the student’s ability to play by the rules of the system.
While a grade can represent how much a student has learned, I don’t think it always does. For example, when a students gets a better grade on an essay than he (or she) did previously, he may have gained better writing skills, which will likely stay with him for his entire life, or he may have simply gained a better knowledge of what that specific teacher is looking for. In this case, it is not necessarily his writing that has improved (although through practice it might have) but rather his ability to appeal to his teacher. Math is another area where much is taught but little is gained. Students may study hard for one test, and they may understand the subject matter very thoroughly, but a month later they may have forgotten it completely. I remember that in Trig/Precalc we learned many trig identities that I then had memorized. I received an A in that class, but I have since forgotten nearly all of those identities. I knew them at the time, but I surely forgot them very soon after the test.
The problem is, I doubt there is much of a better way to measure learning. What I do know is this: grades shouldn’t be a student’s motivation to do the work and the homework. But they are. I will be honest about that. When I’m learning about a subject that I’m not really interested in, I work hard because I want to get good grades so that I can get into a good college. Grades and tests, while inaccurate, are what motivate many students to do their work. And while I understand that that’s wrong, I have no idea how to change it.