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If you have a student(s) who needs to be excused from an Assignment, Quiz, or Graded Discussion, go the Course Grades.
Find the student's name and the Assignment, Quiz, or Discussion.
In the cell for the grade, enter EX (i.e. excused).
Doing so will remove the Assignment, Quiz, or Discussion from the students' total available points, and thus, not count against the student.
NOTE: If you leave a hyphen in a Grades cell, the grade in the Total column will be incorrect. There must be a Grade, Ex, or 0 entered for all assignments, quizzes, or discussions for the Total columns to be calculated correctly.