I don't recall ever breaking out into a sweat after making the sign of the cross, nor being asked to do it a hundred times in a row. (Hail Maries, on the other hand...)
So if there was a religious practice in Christianity which required sufficient physical exertion to make you sweat, you'd be ok with it being taught in public school? Let's pretend for the sake of argument that some group of 15th century monks came up with a dance routine representing some form of prayer (maybe a dance version of the 12 stations of the cross). Would you be ok with it being taught in public school because it also happened to be a good form of exercise?
I'm going to go out on a limb and guess that most people would automatically be opposed to it. I could be wrong, but I doubt it. So, assuming that's the case, what really is the difference other than that you are more familiar with the religious practices and prayers of Christianity than those of Hinduism? Are we supposed to make an exception for religious practices if they're ones that we're not familiar with? That seems a bit irrational as a means to make this determination. So what if we're ok with the yoga moves now, but in 20 years it becomes more commonly understood to be a spiritual practice associated with Hinduism. Does it then become a violation of the separation of church and state?
Is it just what we think something is that matters? Or what it really is? And isn't this all kinda silly to begin with?
I'd be okay with it if it was being taught as exercise, sure. Just like I'd be okay with the kids dancing around to Richard Simmons, even though I'm sure some paranoid parents would worry about their sons turning gay over it.