Well it's May already! You know what that means - June is coming! Instead of waiting until your report cards are due, I suggest collecting all of those overdue assignments now.
Go through your markbooks and make two lists: one by assignment for yourself listing who still has what assignment due, the second for your students by student listing which assignments each student has overdue. Try to make these lists electronically so that they can be called up as the students who have lost their assignments due three months ago are also likely to lose this list. Try not to list all of the overdues on the blackboard as this could be a privacy issue (but, of course, you know your class best).
Also, while you're at it, go through those piles that have snuck onto your desk to make sure you haven't lost an assignment that WAS handed in. Don't feel bad if you find these assignments, just make sure you don't dock marks for your mistake and get it into your books.
Now is also a good time to start entering marks into your report card software if you haven't already. Of course, I would recommend that you keep up with overdue assignments and entering marks into report card software starting in September and working on it at least once a month (the more often you do it, the less work will be piled up to do each time). However, me recommending that in May doesn't really help you. So just do what you can do now.
Here's to an organized May!
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Have great day!