Teachers can still calculate grades after the end of a marking period, because (Report Card) Grading is separate from Gradebook grading (which relies on Marking Periods).  An assignment will be included in a Grade Plan Calculation as long as:
  1. The assignment has a due date within the Marking Period a teacher is using to calculate a Grade Plan Grade
  2. The assignment is flagged to 'Publish to Gradebook'
  3. The assignment is flagged to 'Add to Cum.'
If you need to extend the Grading End Date for teachers to be able to calculate grades, please see: How to extend the Grading End or Review End dates in a grade plan?

For more information on setting Grade Plan Grading and Review Dates, please see: Why isn't a grade plan showing in the review grades drop down?