When it happens...and it will...what should you do? If you're like me, the natural response is to get defensive or want to prove them wrong. In my earlier years in ministry, that is what I did...and it rarely turned out well.
I have discovered that when a parent gets angry, the best thing you can do is ask questions. This allows them to get their anger and frustration out in the open. Asking questions like, "What are you most upset about?" and "Tell me what you are frustrated about" will help you get to the bottom of the issue. The parent will also feel heard, which is the often what they are wanting.
This moves you from being defensive to being empathetic. Remember, your goal is not "win" but to bring resolution.
If you'd like to get better at talking with upset parents, there are more tips at this link.