If you want to remove religion in schools completely, then you have to remove ALL traces. That would mean NO Christmas celebrations or decorations or even mention of it. That would mean removing most of the literature taught (we read the Scarlett Letter in Yeshiva – but public schools wouldn’t be allowed to).
This case seems to be a child sharing what he loved. Does that mean a Jewish kid couldn’t bring in a tallis for show and tell? [not arguing the appropriateness…just the action]