Think of it like this..are you going to CHURCH or are you going to shool? If it is being used as a Church..it is NO LONGER a School is it?
If you lived in WA I would point you to our AG opinion on what is a school. The law (in WA) specifically states "exclusive use for school activities" if it is a marked school building, or not.
Look in your state law for the definition of a K-12 school, as quite a few churches use (lease) school facilities for church on Saturday or Sunday...so, in your mind, is that school facility still a school when it is lease for some other activity?