Here is an idea for consideration:

If a teacher does not want to add an "extra credit assignment" and wants to change the overall grade percentage of a student after "calculation" - create an override feature.

Example - let's say a student got a 91% (A-) for Quarter 1 and the teacher believes this student deserves a 92% (A) for his/her hard work. By using the override feature it saves steps of creating an "extra credit" assignment or by bumping a student's grade in assignments.