Here's the PDF of the text of the decision, Peck vs. Baldwinsville School District, which contains this description of the assignment that was sent home to the parents:
To enhance the student’s understanding of his environment, we are askingstudents to make an environmental poster at home and bring it to school by June 4th. These posters will be on display at our program. The children may use pictures or words, drawn or cut out of magazines or computer drawn by the children depicting ways to save our environment, i.e. pictures of the earth, water, recycling, trash, trees etc. This should be done by the student with your assistance. The poster should be able to fit in the child’s backpack. We hope this project will be fun for all!Oh, it was fun all right!
Here's the mother's description of the first attempt at making a poster:
[S]he and Antonio sat down together one night to do the poster, and she told Antonio that the school wanted him to do a poster on how to save the environment. Antonio responded, according to [the mother, Joanne] Peck, that the only way to save the world was through Jesus. Peck then provided Antonio with art materials and some magazines, and Antonio selected pictures, cut them out, and, with his mother’s assistance, arranged them on a piece of paper. Antonio (who could not read) told his mother what he wanted the poster to say, and Peck wrote out what Antonio said so that he could include the words on to the poster.A complete failure to do the assignment, right? But the boy is only 5. What difference does it make? The teacher tells the mother to do a new poster if she wants it to be exhibited in the cafeteria along with the other kids' posters.
This poster, which was turned in to [the teacher Susan] Weichert, was comprised of the following images: a robed figure (who is described by both parties as “Jesus”) kneeling and raising his hands to the sky, two children on a rock bearing the word “Savior,” and the Ten Commandments. Written on the poster were the phrases, “the only way to save our world,” “prayer changes things,” “Jesus loves children,” “God keeps his promises,” and “God’s love is higher than the heavens.”
[The mother] again assisted Antonio in selecting images (from the computer and from a religiously-themed coloring book), and in arranging pictures on the poster. The second poster depicted, on its left side, the same robed, praying figure pictured in the first poster. It also showed, in the center, a church with a cross. To the right of the church were pictures of people picking up trash and placing it in a recycling can, of children holding hands encircling the globe, and of clouds, trees, a squirrel, and grass.This poster was displayed partially folded over to conceal the Jesus image. There is an interesting fact question here about how the school would generally treat irrelevant images, and the deposition testimony indicates that the child would have been asked to explain how the image related to what they'd studied. With the image of Jesus, the school did not ask and assumed the child's message was that God would save the environment, though it makes some sense to think that religious belief would motivate some people to behave virtuously in matters environmental.
As a matter of free speech analysis, the questions are whether the school engaged in viewpoint discrimination and if so whether avoiding endorsing religion is a compelling state interest justifying that discrimination.
The court does not in any way acknowledge the general problems schools face when parents (and students), fired by religious zeal, look upon every school assignment as an opportunity to send their religious message into the classroom.