Yoga is just an opening for the Devil....
I've done my share of Yoga, and I don't get anything religious about it. But lets examine it a little further.
Yoga originates in Hindu philosophy. Most westerners think of Yoga as some stretching and breathing techniques. It depends a bit on your yoga-teacher how spiritual of an experience they choose to make it. Actually, I have had to choose my yoga classes carefully, because certain nights of the week they have Christian Yoga! Is that an oxymoron perhaps? (Apparently some Christians think so, and have developed their own alternative to Yoga.)
In the end, I'd actually say that it'd probably be better to avoid Yoga in school. While the Yoga we know is far removed from it's religion, just like Santa Clause and the Easter Bunny, and I don't really think that Yoga could convert anyone to another religion (if it does, you weren't that set in your religion to begin with), I wouldn't want it to get turned on it's head and have Dianetics taught in school. In any event, exercise should be taught by professionals for safety. Perhaps stretching, breathing, and exercise can all be pulled out of Yoga and presented in a way that wouldn't be called Yoga and doesn't involve finding you're center.
In addition to that, I don't think that many of those positions would be appropriate for a class of kids hitting puberty. I remember giggling my way through health, and biology, I'm just imagining, if I had yoga in school, being stuck on a mat behind the flatulent kid while attempting downward-kneeling-dog, or worse, being stuck in front of the nerdy kid with a crush on me who's now getting a great view.