We have a No-Skippy Jar where you take a piece of candy if you haven't skipped a class that week. I think the fact that all of your sisters see if you DON'T take a piece of candy is more of an incentive not to skip class than the candy itself.
We also have a Smart Cookie drawing: anybody who gets a B or higher on a test, project or paper that week gets to put their name in a hat to have a chance at winning a prize.