Good question. 先生also can mean teacher, like 教书先生。some old articles (not modern time ) call teacher 先生。but in mainland now, 先生mean husband or sir.

先生 is a title, like Sir, Mr, Mrs, Dr, etc, and it means Professor, or Pro. It is added to the name: 鈴木さん (Mr. Suzuki)；鈴木先生。(Pro. Suzuki) It is the title for teachers and professors of all branches, including masters of martial arts. Being a title, you NEVER use it to refer to yourself! Just like you never call yourself Sir, or Madame. You cannot call yourself "san", "sama", or "sensei". 教師 = teacher or professor. When someone asks you what you do you say: 教師です。 You never say 先生です。

Usually 教師 is used as a general teacher. 先生 is also the meaning the same, but mostly it means "your teacher".

Your question always makes me ponder. ＾＾ 教師 is mostly used to refer to one's occupation. 先生 can be used for it too, but if I were a teacher and to introduce myself, I would say "私は教師です." People will understand if I say "私は先生です" but 教師 sounds natural. But it sounds better with 先生 when you refer to other person's occupation. If I had a friend and s/he is a teacher, then I would introduce him/her by saying "彼/彼女は先生です." Generally 先生 can be used at any time, in any context. :)

Sorry, i just see you are asking Japanese ~