This type of system works for kids of all ages, and it encourages them to go out of their way to be kind to others. You can have "catch-the-kindness" forms available in the office, and if someone (a student, teacher, parent, administrator) sees a student do something nice for another person at school, he or she can fill out a form telling the person's name and what he or she did. For younger kids, the reward can be eating lunch with their favorite teacher or extra minutes at recess; for older kids, a homework pass or a certificate to a local movie theater works well.
Fun idea: Make a bulletin board outside the school's office, and each month (or quarter), put a picture of the "catch-the-kindness" students with a brief explanation of how they were kind to another person.