doctor/teacher

医者(いしゃ)	Medical doctor – normal version

医師(いし)	Medical doctor – formal version. Used in newspapers and documents. You wouldn’t say “I just got hit by a scooter! Call the 医師!”

先生(せんせい)	Teacher, but also any upper-class smart professional:doctor, lawyer, philosopher, etc.

教師(きょうし)	A more formal word for teacher (and, unlike 先生, this ONLY refers to teachers).