Is there a way to hide all previous assignments, whether they are completed or missing? For our younger students, having all assignments from Quarter 1 still showing when they click "Grades" is very confusing and overwhelming, especially when we are asking them to complete Missing Work, but only from this quarter.
You can unpublish them from the Assignments page in the course. That will hide those items from the students' Grades page.
I think you are going to need to delete the assignments from the course. If you do, be sure to back them up, maybe by copying to a Class Master Sandbox, for resuse. Assignments with submissions can't be unpublished. Putting assignments is a locked module or hiding the scores in speedgrader doesn't hide the assignments in the Grades window.