When one of my students tries this my standard reaction is : "Then it's MY art and I will be taking it home". After a few times of this they seem to have been able to figure out their problems. I teach K-6 and I may show them sometimes on another bit of paper that I tend to carry around with me. I also like to check that they have their names on their own work by stating that I will be coming around soon to see if any of MY artwork is around, i.e. the pieces that are not named are ipso facto - MINE. The last minute scurry is a hoot.