In general, we encourage leaving the assignments un-hidden so that students can refer to their work when studying later and so that students have a way of recalling their grades.  We didn't design the hide function to work with assignments that are in active use. If you are hiding assignments to declutter your student’s screens, consider this alternative: prefix the assignment name with something like "Unit 1" and then keep the assignments un-hidden. You can also use the Last Date for Student Submissions feature if you want to make sure that students don't submit after a given date.