This has been used in a variety of ways - this link explains how to use it with different age students and I like the fact the students need to tally up their rewards.
Class credit card reward system
SERC / Newbridge School
Barley Lane Campus, 258 Barley Lane
Goodmayes
Ilford, IG3 8XS
SERC is a service and information centre brought to you by Redbridge.