If you want to set a default grade for a certain assignment, use the assignment drop-down menu. This will allow you to input scores for students who do not have scores or to overwrite already entered scores.
Note: When Multiple Grading Periods are enabled in a course, you cannot set a default grade for any assignment that has at least one student in a closed grading period.
Open Assignment Drop-down Menu
Hover over the assignment title and click the drop-down menu.
Set Default Grade
Click the Set Default Grade link.
Create Default Grades
Type the default grade value in the text box [1]. If you would like to overwrite existing grades for the assignment, select the Overwrite already-entered grades checkbox [2]. Click the Set Default Grade button [3].