In my public elementary school in the 70's, there was ONE

 In my public elementary school in the 70's, there was ONE teacher who went by Ms. All the other women teachers were Miss or Mrs. Even when my kid went to school the 2000's, teachers were Miss or Mrs. I've been using Ms since I was 16. Fortunately, in the business world, I have found people are more likely to default to Ms because you really don't know what a person's status is unless they tell you.

One of my favorite teachers in high school (who was married, in her 50s, and had her husband’s last name) absolutely insisted that people refer to her using Ms. instead of Mrs. She would even politely correct students when they said Mrs., saying that she worked hard for her professional career, and that her identity is completely based on her being her own woman, not “her husband’s wife”