Learning the mechanics of how a custom of indigenous people is performed, even participating in those mechanics, does not "make" a person become a member of a faith group. Given that the kids in the classroom aren't members of the tribe, it's not like they can just "join the club" anyway.
And more to the point, if participating in a demonstration of the culture will cause the children to "believe" in this faith system, that, in itself, means that the so-called atheist is giving power to the ritual and suggesting that the "religion" has a life of its own.
It's as stupid as refusing to go to a wedding because it's going to be in a temple or church. If a person does not believe the religion has power, then the act of being present isn't going to make this nonexistent concept suddenly able to hold sway over someone. Unless, of course, the person is wrong, and the religion is "real." A blob that will overtake them, and leave them powerless to refuse!
It's role playing--a way to learn. Those parents should home school their kid if they don't want the child to learn about how other cultures live. The purpose of the explanation sent home to the parents in the school letter is so the PARENTS will know what the purpose of the lesson was, in the event the children do a poor job explaining it to them should it come up. And if they sent a letter home saying what was going to happen, I should think the parents had enough time to opt out.
This is not the same as learning about the varying forms of Christianity, Islam or Judaism. either. Those religions have big box houses of worship scattered hither and yon that are readily accessible, and some have TV channels, too where people can get that information.
You've pretty much got to go to the rez and/or associate with a tribe to see these sorts of rituals close-up--though YOUTUBE might have a video or two, if you know where to look.
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