I don't know the rules and regs that apply to the US, so I'm not a lot of help here.
I know this is going to sound really crazy, but - dress the part. Something that has helped me when I have a difficult task and I fear I'm facing a fair bit of opposition - I go back to my uni dramatics days and look on it as a role in a play. I dress the part - no power dressing here, plus nothing too expensive or flashy. No ornaments. Nothing in your clothing to upstage you - no nonsense, plain office clothing, muted/dark colours. Make-up - minimal, nothing too bright. Emphasis on the eyes more. Basically, dress like a junior lawyer. Think like a junior lawyer. Use your voice - DO NOT speak with a rising inflection, as if you are asking them a question. YOU are making statements. And if you're concentrating on all this, and have your notes and your agenda ready, you should be too busy to get angry and lose focus.
If you MUST lose your temper, try and stay in character - you are the boss of a business and your underlings have severely disappointed you. There are ways to allow yourself to lose your temper which can be not only therapeutic, but effective. You don't yell, you don't raise your voice, you don't pound the desk. You make sure you have well-prepared, ahead of time, an effective ultimatum. Such as, "We've already had a meeting on this topic and despite this you are still failing in the most basic areas. If you can't get even these simply things right, how can we trust you with the very special needs of this child? If you continue to fail in this way I will have no alternative but to go to your superiors as well as the congressmen, as well as the media, in my efforts to get things fixed properly, once and for all. it's your choice."
Whatever you do, try to NOT fall apart and burst into tears. It puts them in the position of comforting you, the overwrought, emotional, unstable mother - unstable like the son, they will think. It's the last thing you want.
Something that helps me if I feel myself beginning to get angry - I breathe slow and deep and make my voice quieter and lower. If you're not making yourself heard, this actually works to make them quiet down so they can hear you. easy child 2/difficult child 2 uses it when handling a room full of unruly kids.
Keep in mind - these are fairly nice, ordinary, average people who simply don't understand. They don't even understand they need to make any effort - they are simply puzzled by your annoyance over this. They are not out to get you, or to get difficult child (apart from maybe one or two who want the problem to go away). There MAY be an element of "If we don't cooperate, she will take him to some other school and any effort we put in will be wasted, so lets begin by not bothering now."
You need to make clear - "This MUST be rectified, because you WILL have to sort this out if not for difficult child, then for the next child like him. The law of averages says there is at least 1 in every 200 students." (I'm using my Aussie figures for high-functioning autism in Australia - I know it's not his diagnosis, but a kid with that diagnosis will present sufficiently similarly for you to use those numbers if you want).
This is a human rights issue, as far as I can see it - he IS bright, but he still has a diagnosis and an IEP. There are reasons for this. It is a legally binding document and not to be ignored. Ignorance of this is no excuse. Deliberate refusal to acknowledge the IEP is negligence. It's probably a lot of other things as well. It IS actionable - do they want difficult child to sue them later on? You are taking detailed notes, constantly - he will certainly have enough material, if he chooses.
Hopefully there will be others along who can better advise you on the things you really need to know - all I can tell you is what has worked for me. You need the more specific legal stuff now. If you can also find an advocate (for future use, if not for tomorrow) it would be a worthwhile use of your time.
Good luck. Don't let them deflect you.
Marg