E.P.
I'm not sure if you are looking for an awards program that would be individual-based or classroom based. This would also depend on the age groups of the students, too. What works for the younger, won't work for older ones.
I know my children have always enjoyed friendly competition between classrooms. The promise of winning a food-type party, pizza party, ice cream, popcorn party is always fun. Or...even combining classrooms for the team effort - if you reach this quota, all of you will have x-prize (movie and popcorn, party in the Gym, homeroom time becomes recess time outside...etc...)... For a smaller group (definitely less than ten) Depending on how much they LIKE their Principal and how many kids would lunch at one time, would depend on whether they would think it was a treat to have lunch with them! Also... if, by chance you would KNOW a local celebrity (i.e. Baseball player, football player, disc jockey, author, even Jim Nesci's reptile show, ...etc... ) someone who may WOW the kids for the chance to meet! This could be a motivating factor for a group effort!
Not sure how much money you have to spend - older kids like "homework" passes and the school may work with you on that. Bigger monetary prizes are: I-Tunes gift cards, Best-Buy, Dunkin Donuts/Baskin Robbins, Game Stop, Target, or a local fast food place of your choice,
Hope I've touched upon a topic that helps.