Make a simple set of necklaces for each center with that centers name on it. You can just write out the names on thick paper, laminate them, and punch a hole in the top. You may want to use different colors for each center as well. Put a string through the hole and tie the ends and you have a necklace. Make the number of necklaces for each center dependent on the maximum amount of students you want at that center. Hang the necklaces in an obvious spot at each center. As a student goes to a new center they will put on one of the necklaces. This way if all of the necklaces are gone, then your kids will know that they have to choose a different center. You will also know where everyone is working (or where they're supposed to be working).