Yoga poses originally evolved as a type of worship of various Hindu deities, why would a Christian want to open themselves up to committing idolatry and whatever negative spiritual influences that may arise from that?  I'm sure some Christian practitioner of yoga are just naive in thinking it's just about exercise, (when I was a fallen away Catholic, I did it once, not knowing any better) but sadly many have been led to believe anything goes. St. Paul's warning to avoid idolatry in 1 Corinthians, chapter 10 is instructive. It definitely shouldn't be offered as a course at a Catholic school. It seems like it will still be offered but just called something else.