Brandy, I wasn't aware that this was a private high school. Since it is a private school, there would be different criteria based on how they're set up.
In my opinion, I would ask to see a handbook and check out bullying/intimidation. In my heart she's definitely crossed the line on that.
I would point out to the person that you're concerned about the bullying/intimidation. In addition if this parent isn't able to be confidential (she's proven in two classes that she can't be) that she shouldn't be subject to seeing grades. I would ask the question can you see her daughter's grades? Of course not. Then why can she negatively comment about and be able to see your son's grades? In addition, if the teacher is preparing lesson plans and doesn't review the student's work and/or tests how does he/she know where a problem lies and the solution to fix it?
Lastly, if this a private christian school where are the christian standards and why isn't she being made to adhere to them.