In our community, reminders to return library books are included in parent invitations to all of the various end of year celebrations like field day, moving on etc. We even have a decorated drop box for people to return books at these events - even if they don't bring the book to the event, it reminds parents to ask their kids if they have any books still out.
I work with the classroom teachers to get all materials returned. Teachers in some grade levels will compete against other teachers to get all items turned in. Some teachers will give "Bulls stamps" (part of PBS program) for kids who return all their books.
I am wondering how soon other librarians in the state stop circulating to students. For example, my last day of checkout to students is May 11. All items are due May 18. The students' last day is May 31. I am in an elementary school with 531 students. I would love to know how others handle these end of year decisions.
I work at the high school level. I set days for check outs and returns for seniors (they get out earlier) and underclassmen and I sent notices. I also start visiting the class rooms and pull students out to go to their lockers to retrieve past due books. This is my first year out of seven that I had 100% return or payment of books for seniors. The key is to start early. They see me coming down the halls and say "I know Mrs. Pernau I will have my book back tomorow"