Block past due assignments make it as view only.

Thank you Khan Academy team. I am a long time user of Khan Academy . My students benefit from Khan Academy . Thanks for making instructional strategies easy to implement for my students and motivate them , and the easy scoring methods available. 

Please help me with the following concern : 

How can I make  past due assignments as "view only" for students ?. I really like the assignment set up : assigned date and due date . I want to block students from doing the past assignments while they are overdue.  All students should be able to view their score and assignments but not to be  doing all  past assignments at the last week before grade book closes.  Please let me know how I can make View only for long past assignments. (not to remove or archive )

Most of my students are on task with each topic/assignment and retake assignments to improve their score to 100% before the due date. Some students wait till the end of marking period doing it just for the sake of getting a grade. This will not help them learn the content/topic for each lesson to improve prior skills . Moving from one topic to next topic should be in parallel to the assignments and students should move from one topic to the next as to grow and retain concept.  Waiting for your replay.

Thanks.