Penis corner.

A more elaborate and good-natured version of simply scrawling a cock on your neighbour's work. Cut a corner segment of blank paper and add your crudely-drawn phallus. When your classmate's back is turned, place your corner of paper over the corner of his work, with a carefully-placed ruler hiding the join. After your friend has noticed the ruination of his work and let fly with a suitable outraged outburst, you can slide the paper away and reveal that it was all a joke. Relief generally diffuses anger, and a jolly good laugh is had by all.