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.
Unlike what one might expect from its name, an open house at most schools is not meant to be a public event, but rather, it's designed exclusively for the parents and relatives of pupils who are currently enrolled or thinking about enrolling in the school.