I understand why this system was made. I made sense at first, trying to get a general idea through all of the student's work of how well they did.
However, there are some major problems.
First off, the teachers. This is a major problem no matter what, but it's especially bad in the grade system. Each teacher has a different point of view. One teacher will give you, say, 17/20 on a project, the other may give you 9/20. It depends on a teacher's point of view. Simply put, the teacher you have has a major significance on your grade. The sad thing is, in public school, before college, you're stuck with the same teacher(s), so if your teacher sucks, tough shit. In college, you have a better chance of finding a decent teacher since there may be several to choose from. However, it's a guessing game, and even then, there are some classes that only have one teacher available.
That and I must say that the public schools otherwise teach only one method compared to the many other methods out there, and I've heard that only a fourth of students learn the way that public schools teach. However, I've lost the statistic, and I cannot find it again. It seems believable, even though the statistic is also likely incorrect, that most students don't learn the way public schools teach either way.
Second, there's the cheating factor. Granted, if people are caught cheating, they're punished. However, there are still people out there who never get caught cheating, thus get high grades. For college, which is supposed to teach people skills for careers, I have to wonder how those kinds of people would be at the jobs they "trained" for if they cheated the whole way through, or how a high schooler would perform in college. This brings me to my next point, which is why most students cheat through school.
Third, both parents and teachers put way too much importance on the system. We go to school to learn. Why the issue of just plain doing well? It's like a more like a competition than a knowledge-seeking method. The common belief seems to be that: A=Great, B=Okay, C-F=Awful. I've heard from my girlfriend's mom that even a C used to be considered pretty good, so it seems to me that the standards have become much higher for some ridiculous reason. A lot of people seem to have this mentality along the lines of, "We don't care about what you have to do, what you learn, or how hard you have to work, just get that A and you'll be fine with us." Does me getting an A in a class always necessarily mean that I learned anything, and even so, does it mean that I'll remember what I learned for an extended period of time? I've seen how public schools teach nowadays and it sickens me sometimes. Yes, let's learn about a lot of math that we'll mostly never use later on in our lifetime. Get an A for learning useless shit you won't need to remember. Also, let's learn about random facts about a random fish that we most of us also won't need to know about later on in life. Here's an idea: teach these to the people who want to know. Teach the extremely complex theorems in math to a person wanting to become a mathematician, and teach people who want to become marine biologists about the fish. Why are teacher's in public schools teaching a lot of their students about shit like this when they really don't need it? Why not just teach the students something useful instead of overflowing their heads with pointless bullshit?
It's late, I'm tired, and I can't remember if there were any more things that I wanted to say. I've covered my main points, though. In general: the grade system was a good idea on paper, but, after some trial and error, it's showing signs of flailing, and we need to use a new system. I don't know what, just something that's hopefully better than that.
EDIT: I've thought of one way: how about giving a fucking reason why a person got a grade (or whatever else) that they received. Otherwise, we just have people second-guessing on why we got that D+. It also makes the grades more easily contestable, so if the grade is bull shit for whatever reason, you can discuss it with your teacher, or the school board if it gets bad enough.
Which reminds me, parents, if your kid does get a "bad grade," please consider what I said above before discussing the grade to your kid.