Some parents in Encinitas, CA, are upset that Yoga has been introduced in the public schools. They claim that having all children practice Yoga in school is akin to indoctrination into Hinduism. See this article.
The argument demonstrates the difficulty some people have in defining religion. To some people Yoga is not religion, but to others it is. The wikipedia entry on Yoga links it with Hindu philosophy. Unfortunately, the author of the article does not address the parents' claim about the link between Yoga and Hindu philosophy, which is a gaping hole in the article.
There is also money side to the story: the school accepted a $533,000 grant to do the Yoga in the school, and the school does not want to give up that money. It is clearly in the school's interest to say that Yoga is NOT religion so that they can keep the grant money. Just as it is clearly in a Yoga business's interest to say that Yoga IS religion to avoid paying sales taxes (see this case from a couple years ago).