Hand out pieces of string or rope to everyone.
Without naming names, think about if there is someone you need to forgive (it might help if you give some suggestions of situations to help them.) You might have someone in your family you need to forgive, or perhaps a friend or someone at school.
For every person or situation that comes to mind, tie a knot in your string/rope. Explain that this is what unforgiveness does to us, it makes us all twisted up inside.
Knots can be hard to untie. It can be hard to forgive. It takes a lot of courage.
Pray and ask God to help us to forgive.