It's obvious that you haven't actually read the case you cited in support of your argument.

Instead, Kennedy wrote an opinion that, while carefully circumscribed, squarely repudiated the school district's main arguments. He found much wrong with Principal Lee's decision to give the rabbi who was planning to offer the graduation invocation a pamphlet on composing prayers for civic occasions:

"Through these means, the principal directed and controlled the content of the prayers. Even if the only sanction for ignoring the instructions were that the rabbi would not be invited back, we think no religious representative who valued his or her continued reputation and effectiveness in the community would incur the State's displeasure in this regard. It is a cornerstone principle of our Establishment Clause jurisprudence that it is no part of the business of government to compose official prayers for any group of the American people to recite as a part of a religious program carried on by government, and that is what the school officials attempted to do."

Kennedy also noted that the nonsectarian nature of the prayer was no defense, as the Establishment Clause forbade any prayers in public schools, not just those representing a specific religious tradition.

Wikipedia

The facts in the case are that the school district controlled the content of the rabbi's invocation and expected him to offer a prayer. What do you think "invocation" means? And what would any reasonable person expect of an invocation given by any religious professional?

You seem to be thinking up the same arguments that the school district tried to use, and the Supreme Court, including a conservative justice, kicked their ass. Read the Wikipedia article for more detail about the decision. It wasn't made up out of whole cloth, it was carefully researched, including examination of the writings of the Founders and the various drafts of the First Amendment that they considered. This isn't that hoary old cliche "judicial activism", this is strict constructionism--the Establishment Clause means what it says, and the Supreme Court reaffirmed it.