A classic Michael LaFosse butterfly, adapted specifically for folding from a US dollar bill - it takes advantage of the designs on the bill to create markings on the wings, with a color-change for the body, too.
Money Origami Kit, by Michael G. LaFosse and Richard L. Alexander, p. 26
One US dollar (if available.) Otherwise, thin, strong paper of proportion at least 2:1. (Actual dollars are close to a 2.35:1 ratio.) If you're less experienced with money folding, or with folding from small paper, for the class you might consider using something larger, just make sure it's at least 2:1 proportioned! The design takes advantage of the decorations on the bill, though, so it's recommended if you're up to it!