Here's two ideas.

Colorful Popcorn Balls from Taste of Home Magazine

9 cups popped popcorn
1/4 cup butter or margarine
1 package (10 ounces) large marshmallows
6 tblsp fruit-flavored gelatin (lime for St. Patrick's Day)

Place popcorn in a large bowl; set aside. In a saucepan, melt butter and marshmallows over low heat. Stir in gelatin until dissolved. Pour over popcorn and toss to coat. When cool enough to handle, lightly butter hands and quickly hsape mixture into balls. Yield: 1 dozen (I would think that the teacher could also just press into a pan and cut them rather than get the kids messy)

You could also just color the butter green with food coloring.

I would also check out Kraft foods website to see if they have any easier jello/popcorn recipes. It's www.kraftfoods.com.