I had a friend, otherwise OK, who was absolutely convinced that his son was mistreated by one of his teachers in I think being denied entrance to advanced classes. Now even if true, maybe the teacher is a jerk. Maybe he didn't like your son for non-race related reasons.

He sounded so convinced of himself that I didn't bother pushing back. I'm sure the answer to how do you know it was about race would be "because he had no reason to hate my son". Guess what, people hate others for no reason that you're aware of.