Grades in a class are not income. They should be an incentive to maximize learning and performance.
A class grade is simply the score the instructor assigns to the amount of material that he deems that the student has learned.
I used grades in this way when I was teaching. I clearly indicated that the class graded would be ¼ assignments, ¼ tests, ¼oral performance and ¼ final exam. Oral performance was based on responses to questions I asked every student in class most days. Students who were absent without a valid excuse received an F for oral performance. I allowed students to correct errors on assignments for two weeks after the assignment. I passed out a grade summary every two or three weeks. I tutored students individually or in groups during office hours. Most did not take advantage of this, but some did.
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I did not "curve" grades, ever. After ten years teaching I know what a student could learn if they studied enough.
Grades are NOT the same as a salary. If you receive a salary from an employer, you have more money, and he has less.
If you receive a grade from a professor, it should indicate that you have learned something. The more you learn, the higher the grade. But at no time does the professor know LESS.
To a student who thinks it through, a grade is not a reward as a salary is, it is an index of how much he has learned, and an ongoing incentive to improve.