Contrary to what its name might seem to imply, an open house at the average school is not intended for general members of the public, but rather, specifically, for the parents and family members of the students who either attend or are considering attending a school.
Contrary to what one might assume from the name, an open house at most schools is not meant to be a public gathering, but rather, it's intended for the parents and relatives of students who are currently enrolled or thinking about enrolling in the school.