★The difference between 教師 and 先生

先生(せんせい) = a teacher

教師(きょうし) = a teacher

Apparently, those words mean the same thing. 

So what is the difference?

教師(きょうし) is used as a job title.

When teachers introduce their job, they say 教師(きょうし).

They use 教師(きょうし) when they write their job on their resume, contract, etc.

You might hear your teacher say,

私(わたし)は日本語(にほんご)の教師(きょうし)です

I am a Japanese teacher.

先生(せんせい) is also used as a job title, but you can use it for teacher, doctor, professor etc, while 教師(きょうし) strictly means "teacher". It is commonly used when you address a teacher like “Mr” and “Ms”. 

When you want to call Mr. Satou who is your teacher,
さとう先生(せんせい)Mr. Satou

When teachers introduce themselves, they never use 先生(せんせい). 
It's like you are calling yourself Sir, or Madame lol

★When you call your teacher, use 先生(せんせい)
★When the teachers call themselves, they use 教師(きょうし)