I have 5 students who each takes a different number of courses. For each term, each gets a grade of A to D. Now after running for a few terms, I want to know collectively which one gets a higher overall grade. This is what I think:

As Grade A earns 100 points and Grade D earns zero, I assign 100 to each A, 67 to each B, 33 to each C and 0 to each D. Then for each student I add all the points together and divide the sum by the total number of data count. This is the overall grade in terms of a number. The one with the largest number has the highest overall grade.

This is my layman way to tackle the matter. Is there any statistical pitfall? Is there a more sophisticated way to do it?

Thanks.