If you look through the Bible teachers are mentioned a number of times. Jesus was often times refered to as a Rabbi which is the Hebrew term for teacher. A teacher is also one of the five fold ministry that was provided to train the church.
The Teachers ministry comes into effect after the Pastor has gotten the sheep up to a functional level. The sheep can act even when new. Christians should go properly trained because that is how it is supposed to work. The pastors prepare the sheep, and when they are ready to eat the meat of the word they are ready for the teacher.
The teacher has a duty to train the sheep in why they believe what they believe as well as what they are supposed to do for the Lord. This will include the proper functioning of the church as well as how to reach people out of the church. They must be taught how to walk in the gifts of the Spirit and the fruit as well. Many people do not understand how the fruit work so they do not flow in those. They must be taught how to function in that way, because the people who are supposed to have trained them did not give them the tools they needed.
The rabbi or teacher is also the one who knows the Word from top to bottom, precept upon precept, line upon line. They can give explainations so that the Word becomes clear to the hearer. The teacher lays down the Law. There is a reason that only the teacher is given the admonition that if they teach the sheep into a way that leads them astray, it would be better if they had a millstone tied around their necks and they were thrown into the sea. They are to be the authority on the Word. They are to be listened to and followed, they have more power over the sheep and how they grow than any other, so they need to know the word and guard their toungs better than anyone.
A teacher is a talent that is recognized many places, but is not managed properly. The adults need to be taught as well as the children.
The mission of the teacher is to teach the what, the why, the how and why it matters.