Discrimination (from race to religion) is the top reason students are bullied in schools. Set up an activity about tolerance that will get the message across. The "Cross the Line" exercise is a radical way to make people realize that we're all not as different as we think.
Somewhere within the room, create two lines that will mark off two territories.
Ask all participants to stand on one side of the line. As caller, stand apart from the group.
Tell the group that anyone who has ever been discriminated against because of the way they look should cross the line. Have the people who crossed the line face the other participants.
Have everyone return to one side of the line.
Repeat the process several more times. Each time changing the criteria for line crossing. Some ideas you could use would be crossing if you were treated unfairly based upon:
the way you talk
someone you're friends with or related to
Wrap it up with a finishing line. For your last point, say, "Anyone who has crossed the line in the course of this exercise, please cross the line."
Lead a discussion. Participants will have hopefully seen that they have all had similar experiences in their lives, thus promoting diversity and some type of understanding.