I believe your question needs to be approached according to what you mean by "teacher" as in a proper teacher or just someone who expressed their views or observations and such to other people who would listen and take advice or knowledge from that person and put it into practice.
Explanation: It just stands to reason that this first " Teacher" would have been someone who was of course taught by no one. They just had some thoughts or knowledge that they shared with others and had an audience willing to listen and practice what they had learned. The word " Teacher" it self could have have been invented by anyone that realized that there should be a name for people who seemed to have the ability to express their knowledge to others . All in all, the first teacher would not need a teacher.
