Did the principal know about Zach being abused?
Upon questioning by Kimball, Zach's mom asks him not to get the school involved, which implies that they don't know of the abuse already. But when Kimball confronts the father outside the school, the principal has the slightest hint of a grin on her face (I see it, anyway). Then in her office, she of course asks Kimball "What did it feel like to hit that son of a bitch?", which confirms that she was satisfied with seeing this happen.
Did the principal just get a sadistic kick out of witnessing an assault in front of her eyes, or did she at least have suspicions of the abuse?