How is it that a Person Praying at Commencement is an Establishment of Religion?

One of the things that always baffles me every year is around commencement time.  How have we gotten to the point where we accept that a prayer in a single meeting is considered an establishment of religion?

I mean, the Congress of the United States opens with prayer, and they are the ones actually mentioned in the First Amendment as ones that are not to establish a religion!  How can someone, especially a student in a school, be interpreted to be establishing a religion?

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