The "legal" definition of "school" in the WI statutes (in regards to GFSZ) seems to me to be pretty expansive.
948.61(1)(b)
(b) "School" means a public school, parochial or private school, or tribal school, as defined in s. 115.001 (15m), which provides an educational program for one or more grades between grades 1 and 12 and which is commonly known as an elementary school, middle school, junior high school, senior high school, or high school.
If in this situation parents are teaching children in addition to, or other than their own, at a place other than their own home an aggressive prosecutor might be able to make an argument, and have a lot of people attest to the "fact", that it is a "school".
There is no requirement in the statute definition that the teachers be paid professionals or that there be some monster administrative bureaucracy to be a "school". All that seems to be required is "...provides an educational progarm..." and "is commondly know as a ... school..."