What is the duck test?
To paraphrase Dr. Douglas Brooks, until more recent times, Ashtanga Yoga has passed the duck test of being religious --- "scriptures, concepts like revelation and ultimacy, moral imperatives," an [expert] who looks like clergy or a shamanist or an expert of a kind, "mysticism, pilgrimage [to Mysore], duck, bill, feathers, waddles, it’s a duck."
I'm not saying Ashtanga Yoga is only or merely religious, but on first glance to, say, ardent Christians --- I can see why they're aggrieved it's being taught in public schools.
I believe they're right in their claims, too, despite my feeling that students would benefit greatly from the practice.
I would feel as uncomfortable with non-denominational prayer, too.