My yoga class isn't spiritual at all, it's all about breathing and posture and trying to keep up with our super bendy 67 year old instructor! If you search you may be able to find a Christian teacher. There are strains of quite spiritual buddhist inspired yoga classes, and these may not be best for you. No yoga teacher will mind you asking questions about what is the basis of their practice.
You do yoga for you, to get stronger, more centred, to learn about your body and where it's at. You don't need to answer to anyone but your Father, so I wouldn't worry what anyone else says to you, but perhaps reply that you have prayed about it, and are very comfortable with what you are doing, thanks very much
