Why do you think the Lincoln writes such a letter to his son's teacher?
There are some specific challenges in fully understanding the motivation of Lincoln to writing such a poignantly powerful letter to his son's teacher. I do think that we can examine his own background to help provide some substantiation that can help to explain the purpose of the letter and shed some light on the complexity that is Lincoln, himself. Consider his own background as part of this. Lincoln had not enjoyed formal schooling. Lincoln had not experienced the "traditional" methodology of schooling. Yet, Lincoln demonstrated a passion and zeal for learning that was based in the classroom experience and yet transcended it. This is seen in the letter repeatedly when Lincoln essentially asks his son's teacher to provide the content experience, but also prepare his son for a journey that is more than the mere classroom:
Teach him if you can the wonders of books, but also give time to ponder the extreme mystery of birds in the sky, bees in the sun and flowers on a green hill.
The idea that learning exists both in the classroom and outside of it is something that Lincoln understood in his own life and sought to impart to his son through the letter to his son's teacher. Additionally, consider that Lincoln saw the difficulty embedded within indecision and doubt. Lincoln's father was described as a man who lived in "instability and indecision." It can be presumed that Lincoln saw first hand what these realities do to human beings. It is for this reason that he want something different for his own child, something that he conveys to his son's teacher:
Try to give my son the strength not to follow the crowd when everyone else is doing it. Teach him to listen to every one, but teach him also to filters all that he hears on a screen of truth and take only the good that comes through... Teach him to have sublime faith in himself, because then he will always have sublime faith in mankind, in God.
Through his own background, it can be presumed that Lincoln wanted a life different for his son. The foundation to make clear decision and reside decisive action is what Lincoln wishes for his son. He understands that his own father's failings should not be revisited in his grandson. In bringing this out to his son's teacher, Lincoln makes it clear that part of the teacher's aspiration should be to mold honor and character as much as embed content learning within their students. Certainly, this is the hope for Lincoln as a parent to child.