this is something that has been debated before and noone has every really found the answer to it.
I recently started a junior yoga group, not the same i know but there was some debate as to wether i was allowed or not, so I phoned the county PE advisor.
She said that the decision lies with the school, if the head deems you responsible then you can do it.
As I say its different because there is no equipment involved but a quick phone call to the PE advisor should sort it out for you
