Most “wall of separation” cases related to public education involve questions of “establishment.” When Ms. Magdalene puts up Christmas decorations in her classroom, that violates the Establishment Clause. Inviting local clergy to open graduation ceremonies with a brief prayer is a no-no because it’s “establishment.” Requiring equal time for Creationism when it’s time for the chapter on Evolution? You guessed it – that’s “establishment” as well.
From time to time, however, a case will work its way through the system asserting the opposite. In these “free exercise” cases, the claim is that the state – in this case, manifested as the public school system – has hindered personal expressions of religious belief or behavior without sufficient cause. The “sufficient cause” part is important because the state has the right to place some limits on how faith is manifested when there’s a good reason. (Human sacrifice, for example, is a “no-no” even if your gods demand placation.) Government entities must demonstrate that they have a good reason for their restrictions, however. And, if there are less-restrictive ways to accomplish those goals, they have to try those first.