It might be useful to incorporate some alphabetical symbols into the grade books. Symbols like M for missing and E for excused - which then produced an appropriate calculation in the gradebook (i.e. 0 for an assignment marked M, and not counting an assignment marked E) - would really be helpful for keeping track of those kinds of assignments. It'd also be more efficient than having to double-click the assignment score, as you do now, to figure out if a past assignment was missing or if a student was exempt.