In first grade I learned to read well and do arithmetic. In second grade I did more arithmetic and very simple multiplication and division. In fifth grade I learned some of the typical writing approaches that got my thinking on how to write. My vocabulary and technique has changed little since then. Grades equal learning.
Harder work equals better grades.
Therefore, work hard and you will learn more. Grades don't necessarily measure anything but effort minus mental disabilities. One could learn a lot and be bored by the repetive practice, leading to lower grades which discourages the student from even learning at all. To elaborate, let me point out a few of the facts on grades. Grades are not comparable on what they have taught students.