Incentive effects of prizes on children

This is in K2's primary school. The classroom that collects the most boxtops gets a prize. Along the course of the year, the teachers realized that kids were starting to steal box tops from one classroom and claim them for their own classroom. Zip lock bags used to be taped to the door of their classrooms so that parents can just drop them off whenever was convenient. Now, the box tops are kept by the teachers and submitted by the kids.