Last year my school had class contests to get everyone motivated. It really worked well, and like you, it was a teacher that oversaw the contest.
Anyway, each class had a container in their room and they turned in their boxtops. Each week a volunteer counted up each class' boxtops and an announcement was made about who was in first, second, and third. Then at the end of the month, the classroom with the most boxtops won the 'party' for that month. The parties varied each month. Some of them were: hot choc. and cookies (winter), ice cream, frost your own cupcake, popcorn, and some others i can't remember. Anyway, our teacher worked with our PTO so they paid for all the parties and also put on the parties.
Maybe this will work for you. Good luck!