Butterfly Gardens are quite popular these days. Many large Museums and Zoos now have a butterfly garden. For Earth Day consider having students create a butterfly garden on the School Grounds.
This poster will be a great guide for what to plant, the reason for planting and a few of the butterflies they can expect
Remember you will probably download this post 8.5x11 size but it can be taken to a copy shop likes Staples and blown up to poster size.
John F. Gardner
aka - Environmental Ed