In Korea, it is not polite to address a teacher by their name. So for example in this west, we might call a teacher Ms. or Mrs. Johnson/Mr. Johnson, but in Korea, the teacher will literally just be called teacher by the students.
So if a student raises their hand to ask a question, they would start of with “선생님…”. This is really no different than other parts of Korean culture and language because most people in the workplace are addressed with a certain title, not by the person’s name.
As written in the graphic, 선생님 is the most common and formal way of saying teacher in Korean. For a more casual/informal way to call a teacher, students might use “샘/쌤”. This is often used by students who are really comfortable and have a good relationship with their teacher.
Other vocabulary in this infographic:
교장 선생님 (gyo-jang-seon-saeng-nim) = Principal
교감 선생님 (gyo-gam-seon-saeng-nim) = Vice Principal
부장 선생님 (bu-jang- seon-saeng-nim) = Head Teacher
교수 (gyo-su) = Professor