The following activity is from a classroom management book called "Tribes, A New Way of Learning and Being Together" by Jeanne Gibbs. A teacher friend of mine told me about this and I try to share it as often as I can.
Have the entire class sit in a circle on the floor. Take a large sheet of paper (like an opened paper bag) give it a name (one that is not being used by anyone in school such as "Herman".) Tell students that they will pass "Herman" around the circle and when "he" comes to them, they will say something hurtful (no swearing) to Herman and make a crease in him. Continue to pass Herman round and round the circle, folding "him" with each hurtful remark. Eventually Herman is completely crumpled. Now the teacher explains that this demonstrates what happens to someone who is repeatedly bullied, they collapse inside, into themselves. Now have the students pass Herman around, as they do, each student says something positive to Herman. With each positive remark the student unfolds a crease. Have each student continue passing Herman around, saying something positive and unfolding a crease. Talk about how the sending of kind messages helps to heal the damage done from bullying. After the students have finished giving positive messages and unfolding creases, Herman will be completely unfolded and laying flat. Students will notice however that Herman still is not as good as new, that the creases still show where there were once folds. Explain that this is because bullying leaves scars inside a person, they are always there and never go away. People who are bullied never forget the incidents, the damage stays with them for life. Students have a strong, empathetic response to this demonstration. The teacher I know displays the crumpled paper on the wall by the door. He points to it occasionally when a student acts in a way that is hurtful to another. He said that previous students have come back to his room just to visit "Herman", long after they had moved on to the next grade!