Hello! I am a licensed attorney, admitted to practice in state and federal court. I have a nearly 100% satisfaction rating so all that means is that you can count on me to help today. Generally, under the U.S. Constitution, there must be a division between "church and state" meaning that a public organization, like an elementary school, may not promote nor inhibit religious activities. In your case, there is not going to be clear, bullet-proof guidance on a situation like this. One one hand, if it is held during school hours just off school property, then it is likely going to be considered promoting religion. On the other hand, if it is off school property, even if it is during regular hours, and if the school has nothing to do with the religious teachings, don't provide any materials or tools, and don't promote it to the parents, then they're likely not going to be considered to be violating the Constitution. Accordingly, my recommendation would be to consider how much the school is promoting this, if at all. If not, then there is no need to worry. Alternatively, if the school principal makes the suggest that people attend this thing, then it is.
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