As a student, one of the biggest things that I wish Schoology had is a way to tell the difference between a completed and uncompleted assignment in the upcoming list and the calendar. An assignment with a submission box or other measurable "completeness" (test submitted, comment posted etc.) would be completed once that measure is fulfilled. Assignments that are graded but do not have submissions could just have a checkbox or button that marks the assignment as complete or not complete. Visually the difference could be as simple as a color change for completed assignments, but anything that allows completion to be determined without opening the full assignment would be good.
Visual difference for "completed" assignment