For Lincoln in the letter to his son's teacher, I believe that courage is identified with the need to stand for one's convictions or beliefs. Lincoln articulates this in the letter through direct and clear language:
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.
Lincoln seems to understand the need to withstand public pressure when one heeds the sounds of their conscience. Certainly, Lincoln's own life represents this. There is a certain call to courage to both "listen to everyone" while "filter all that he hears on a screen of truth." There is courage needed in teaching such lessons. It would make sense that Lincoln would reach out to the teacher in teaching such lessons to his son, values that he himself would be striving to impart in the boy. The closing line of the letter reflects this. Lincoln closes it with a call to asking the teacher to do "the best you can do" because "he is my son." In Lincoln's understanding, courage is needed in order for this child to become both a good student and even better person.