I know of no proof that academic grades naturally fall into a bell curve or any other normative distribution. Here are a few quotes.

From the University of Illinois Center for Innovation in Teaching and Learning cte.illinois.edu/testing/exam/course_grades.html:

Grading on the Curve
This method of assigning grades based on group comparisons is complicated by the need to establish arbitrary quotas for each grade category. What percent should get A’s? B’s? D’s? Once these quotas are fixed, grades are assigned without regard to level of performance. The highest ten percent may have achieved at about the same level.

Those who “set the curve” or “blow the top off the curve” are merely among the top group; their grade may be the same as that of a student who scored 20 points lower. The bottom five percent may be assigned F’s though the bottom fifteen percent may be relatively indistinguishable in achievement. Quota-setting strategies vary from instructor to instructor and department to department and seldom carry a defensible rationale.

While some instructors defend the use of the normal or bell shaped curve as an appropriate model for setting quotas, using the normal curve is as arbitrary as using any other curve. It is highly unlikely that our college and university student abilities or achievement are normally distributed. Grading on the curve is efficient from an instructor point of view. Therein lies the only merit in the method.

On the invalidity of the bell curve in employee evaluation in the business world, please see this article in Forbes magazine: www.forbes.com/sites/joshbersin/2014/02/19/the-myth-of-the-bell-curve-look-for-the-hyper-performers/

Here is a quotation from an anonymous law professor in Deborah W. Post, Power and the Morality of Grading – A Case Study and a Few Critical Thoughts on Grade Normalization, 65 UMKC L. Rev. 777, 806 (1997):

There is no such thing as a normal class or a normal distribution of students in a class. Anyone with significant teaching experience knows that this is not so, that there are classes with a large number of better students and classes of an unusually large number of poorer students and everything recognizable in between. Normalization fails to give students the grades they deserve

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