From the beginning we know that Jefferson’s trial isn’t fair and we feel helpless as the only defense they can give for the child is “he’s too stupid to be guilty.” When you try to dehumanize the innocent (or at least who we assume is innocent) that says something about the regard you hold for them. Also, the trial, because he was a black boy, was not ‘innocent until proven guilty’ but ‘guilty unless proven innocent.’ And we see this replicated today, with around 43% of people in jail being black, despite black people making up only approximately 12% of the American population. It shows that race based judicial injustice didn’t end with Jefferson.
Also, Grant talks a lot about how Jefferson represents all of his students. His conversations with Antoine about how being a teacher doesn’t really matter, trying to help the kids doesn’t really matter because they’ll grow up to “be the niggers they were born to be.” They talk about a cycle and how Jefferson and Grant and everyone else in their town just fits into the cycle and you can't break it. Jefferson represents the average kid in their town pretty closely.
And symbolically he represents black people as well. The talking about Joe Louis and Jackie Robinson reference how these distant people stand somehow for the people in their town. So if Jefferson too can be a hero then doesn't he stand for the people even more because they know him? If the boy cried out for Joe, will Jefferson cry out for Jackie, will some other black boy someday cry out for Jefferson?