If you think about it, a person's keys tend to go just about everywhere with them, people never leave the house without them. Let your guests take their prom memories everywhere too with a personalized key chain to remember the night by. Choose from a poker chip or a custom key tag of your choice as a token from the evening that will last for years to come.
When you give the entire senior class a matching key chain, it creates a unique bond for them. Even at a large school where everyone may not know each other, if they see someone in town with the same key chain as them, they can smile and wave, knowing they both went through the same thing and attended all the same events throughout high school.