Personal Attack
The comments, when assessed from various angles, were proven to be private. If they were fair comments, Chris admits he would have spoken to his son, but they were far from that. So, the only option was to talk to the teacher in person. In situations like these, it is best to confront a person directly. But given that this was a school, that had a board and its own ways of dealing with controversial issues, the situation was not so clear.
Social Media Intervention
Poor grades by students are inevitable at times, and harsh criticisms are not the solution. Teachers ought to treat all their students fairly and be positive with feedback and help them grow. And, by the way, what would prompt a teacher to call a child pathetic? What was her ulterior motive? Certainly, teachers will be disappointed and perhaps even frustrated when their students perform poorly on tests. But there is a healthy way to convey this disappointment.