Advocacy Avenue #1:
Stay calm. It is a mute point to try to get something if you have lost your cool (crap, etc). No matter how badly you know your child needs a certain therapy, diagnosis, opportunity, etc. If you are out of your mind, no one will listen to any of us. However, we might have the police called (no one wants that). We have to separate ourselves from the emotional impact of the situation. Men are particularly good at this. They typically can see things methodically without emotion. If you have a partner, or if you are the partner that can stay calm under pressure, have the calm one do the talking in public.
Another calming aid that I use (truth be told), is I embalm myself in Lavender Essential Oil before IEP meetings, evaluations, conferences, etc. I can literally feel a peaceful grounding with it on my skin. (You can purchase Essential Oils through Young Living at THIS LINK. You would join my team at that link. I will support you & help you meet all your health goals through our amazing products... if you want more info see: Essential Oils Getting Started.). Put things in writing (delete the bad words before you hit the send button for goodness sake). Emails are so very effective. Email takes the emotion out. It also gives you evidence of communication that you can present to bosses, the supreme court, or whomever. I might type an email while being ragingly angry, shaking, and spewing words that I wouldn't want to hold in my hand. Regardless of my emotional state when typing, hopefully the recipient will look at the valid points I have communicated, and not sense the frail state I was in at the time... please keep reading in the first comment, Darling 👀