The teacher will tell the children whether she wants to be called Mrs. Smith or Ms. Smith, or Miss Smith or Ms. Smith at the beginning of the year. Miss is fine. One of my daughter's teachers last year was a Miss. I don't think any have asked to be called "Ms."
Very young children may make the mistake of calling every adult woman "Mrs." (when you're 4, even a 19-year-old is "old") but the teacher will usually gently correct them and say "It's Miss Robinson, remember?"
- For Teachers