Teachers are also the mouthpiece of God when they teach according to the Spirit; and they also interpret God's will when they interpret the scriptures correctly.
Teachers are also held to a high standard: "My brethren, let not many of you become teachers, knowing that we shall receive a stricter judgment." James 3:1 Teachers and prophets can both be in error and speak falsely.
From what I can tell, your idea of a prophet is someone who gets direct revelations from God. Maybe there are such people; but if so, their revelations are only as good as the scriptures they have to back them up. Your original post identifies the root problem of direct revelation: how to know whose revelations to believe and whose to disregard. That's why God gave us His word so we could easily discern what is true and what is false.