Averaging student grades doesn’t make much sense to me. A lot could be seriously written on the subject, but this post will just be a little rantish. (Note: A lot of my specific arguments here are inspired by Ken Gatzke)
Let us suppose that your class has 5 graded papers, all of approximately the same length and importance. Now consider 2 students, Andy and Ben. Andy gets the grades A B C D F on his papers, while Ben gets the grades F D C B A. If you average their grades, they will roughly come out to a C (assuming the F was within a reasonable range). However, the grades tell entirely different stories about the two students. While there’s no necessary connection between a set of grades and a particular story, let me stipulate some stories to drive home the point.
Andy did well in his previous class in the subject and understood the introductory material very well. However, he gained absolutely nothing from the experience of this class. The further the material drifted from what he already understood, the worse he did. Ben, on the other hand, started out with no background in the subject whatsoever. However, over the course of the semester through hard work and perseverance he came to understand the subject completely.
Now, what would be the justification for giving each of these students a C? It seems to me that by these accounts, Andy deserves an F and Ben an A. And while these are not the stories of most students, Andy and Ben are sure to eventually fall victim to this unjust grading scheme if it persists.
Obviously, resolving this problem would require examining what grades are supposed to do, as well as how to make them do that. As that would be a much longer rant, I’ll do the usual philosophical thing and leave the reader with a criticism and some questions - answers are left as an exercise.