"Teacher" is most commonly used for people who teach in grade school and high school. "School teacher" isn't used very much any more; it's old slang. Professor generally refers to people who teach college classes. As far as I've ever known, no one uses the word "master" to refer to a person. "Maestro" is used to refer to the people who lead and conduct orchestras. (I've also heard it applied to fencing instructors.) In the US, "schoolmaster" is never used; I believe that's a term from the UK to refer to people who oversee the teachers.