My mother-in-law (who is an amazing kindergarten teacher) taught me how to do this project. I thought I'd share it now so you can start saving egg shells and by the time Easter arrives, you should have enough for the project!
Supplies: Egg shells, food coloring, glue, paintbrushes, paper.
Wash out membrane from egg shells and let them dry. Place the egg shells into a plastic baggie and add around 10 drops of food coloring and 1/2 tsp water. Crush and shake the bag to smash shells into small pieces and to distribute color. Repeat using separate baggies for different colors. Leave the shells in the bags over night (for the color to set in really well). Then spread out on newspaper to dry completely.
Decide on the shape for your project. When I did this for the kindergarten class, I found a simple (free) butterfly clip art online and pre-printed it on paper. For the first graders, we drew an oval, egg shape. We talked about and looked at examples of Faberge eggs for inspiration.
The students will "paint" glue onto the desired picture. Don't be shy with the glue. The more glue, the better the shells will stay in place. Next, they will sprinkle egg shell onto the glue... and make a beautiful spring-time mosaic.