The answer to this question is a matter of perspective and what one individual considers a "real teacher." I believe that Jefferson, the imprisoned many who has been sentenced to death, is the real teacher in this novel. Grant's impact was on one man, while Jefferson's influence has the potential to impact an entire society.
When Grant is asked by Jefferson's grandmother to teach him something, Grant is initially reluctant. He does not believe he can do anything to change Jefferson's perspective on life in the little time that he has. Grant does not even believe that he can have any impact on his own students as their teacher. He believes his mission as a teacher in his community is futile just as be believes his mission as a teacher to Jefferson would be futile. He has become jaded over the years, and most readers would be hard-pressed to blame him.
Despite his reservations, Grant agrees to meet with Jefferson because he knows it is what his aunt wants him to do. He continues to be skeptical about any impact he will make, even after continuous visits. Eventually, though, Grant and Jefferson start to warm up to each other. When Grants read's Jefferson's journal, he sees that Jefferson has written, "Good by mr wigin tell them im strong tell them im a man." This is significant because the goal for Grant's visits was to help Jefferson to see that he is not the animal they made him out to be in courts. His grandmother wanted Grant to show Jefferson that he was a man.
Yes, Grant was an excellent teacher for helping Jefferson to see this before he is put to death; however, the impact on Grant is larger and more significant. Now that he has seen the change he can make in one person, he is able to see that it could be possible, after all, to make a difference for the school children he is teaching. Against all odds, just as he helped Jefferson, he might be able to help them, if only one or two. Furthermore, Jefferson's words to Grant acknowledge the importance of the community's awareness that Jeffereson died a man. He knows that the children and the rest of the black community need to see that he was able to make a change in his life, take ownership of his actions, and be a man in the last moments of his life. He is aware that the community's knowledge of this will help them in the long run. This makes him the real teacher in the novel.