May I say something? Since I have a Chinese culture blog here…..:) 

老师 (Lǎo Shī)

师 derives from Tang dynasty scholar 韩愈 Han Yu’s article 《师说》”My Thought of Shi”. It says “师者，传道授业解惑也” “The person of ’师’ needs to propagate the doctrine and their ways to resolve doubts” (You can read the full article here with English translation). The article shows what are need to be a teacher and what kind of people is deserved to be called teacher in Chinese culture. 

For some skills like martial arts, cooking, folk arts etc. we use 师父/师傅 (Shī Fu) which means “the teacher who is like my father” because these skills need to be taught one on one. It can also be used as a polite word for people who have skills and you don’t have those skills under the moment, like a master of martial arts, a monk, a chef, a carpenter, a taxi driver etc. 

先生(xiān shēng)

In 《论语》”Analects” of  Warring States period, the word means “father and brothers”. “有酒食，先生馔。”注解说：“先生，父兄也”。

Later in 《战国策》”Zhan Guo Ce” it means “elders who are very educated”. “先生坐，何至于此。”

So 先生 was used to refer “teacher” in China for a long time because they’re mostly the elders who are considered the most educated.  From the “Book of Rites” (Han Dynasty)  “从于先生，不越礼而与人言。”注：“先生，老人教学者。” to the early 20th century’s 教书先生. Female teachers were also called 先生 like Li Qingzhao.

In the later 20th it gradually became a polite word for male adults. It still suggests “The one who is educated” but not necessarily “an elder” but you respect them like an elder. Translate it into English, it’s kinda like “Mr./Sir”  - 王先生 Mr. Wang. 这位先生 You sir/ this gentleman. But it’s also used to refer someone’s husband. 您先生 Your husband (polite)  我先生 my husband. Since then, using “先生” to refer teacher was gradually disappeared. 

Your Chinese teacher might have told you 先生 means “Mr.”. That’s it. But the meaning and history behind every Chinese word may have some differences from the English translation.