A childhood belief about classes

I went to a 'private' Christian school my first two years of school. When I heard people talking about private school, I thought that meant that people would have to knock on the door before they came into the school. Even after I started attending, I kept wanting to tell people coming through the front double doors, "Hey, you're supposed to knock." I never had the courage to speak it though. I realized about 4 years later what it really meant and that belief has always made me laugh.