As Sandeep (
S Mewara[
^]) already pointed out in his comment to your answer, you need some way of storing the due date of the assignement a teacher (tutor) appointed to a student or to be more realistic to a group of students. Usually a due date would also not be a applied on a per student basis but rather to a collective assignement that is issued during a course. You might find it useful to add the ability that a teacher or tutor might expand the due date for some or all of the students.
Nevertheless you must model your application in a way where courses and assignements with their respective due dates and prolongment of these due dates in general or on a student basis can be persisted. Without some kind of persistance layer all information would be lost when the application is restarted.
Please think over your design and if you have questions or doubts on how to solve this, please do some research first and if all else fails come back with some solid requirements. That way it will be so much easier for us to help you.
Best regards,
-MRB