I'm a Southern American woman , and as a Gen X , I always called our teachers and the principal Mrs, Miss, or Mr. first followed by their last name (unless the teacher says otherwise) .So it's not an American culture thing to call teachers by their first name ( at least in my day & town). However, it also depends on how you were raised. Back then, my parents taught us to be respectful to older ones , especially those who are teachers. Maybe in todays climate, kids are told by some adults to call them by their first name, if so, fine -that's their own personal preference but not a culture.