Part One: The Help
This isn't going to be a long drawn out thing, I'll keep it short and focused with respect to your situation.
Here are the four steps one would take if this were happening to them:
Request a mediator to help you and the school reach an agreement.
Ask for a hearing before an impartial hearing officer.
File a complaint with the federal Office for Civil Rights.
File a lawsuit.
You should begin by demanding a state appointed hearing officer and make sure you have the official paper trail of your 504 plan.
Part Two: The Game Plan
Here's a play by play of what should go down.
Tysun's parent/guarding: *Calls school's main office during business hours* "Could I speak to Mr. Principle please?
Secretary: "Sure thing let me transfer you!" or "He/she's busy can I take a message?"
Don't just leave a message!
Tysun's parent/guarding: "Alright then maybe you can help me." or get transferred to principle.
Tysun's parent/guarding: "My son has been diagnosed with -insert shit here- and has the 504 plan which allows him to -insert shit here- It has come to my attention that Mrs/Mr. Teacher is saying they don't agree with the plan and is consistently failing my son/your name on "late" assignments. I'd like to see what I can do about that with the administration or if I would need to set up an appointment with a state appointed hearing officer?"
Yes that last part was kind of a threat.
From there you'll either get a solution or you'll need to set-up that appointment, I'm sure you can find out on one of the websites I'm about to link you to. If you can't find it and it gets this far, tell me.
Part Three: My Opinion (You don't have to read this part, it has nothing to do with helping you, it was just my take on the situation.)
Of the three disorders you mentioned two are not even officially recognized diagnoses and in my opinion all of them are fake disorders made up by people too lazy to push through and get the things done that need to get done on time. I completely see where your teacher is coming from, but unfortunately thanks to our new snowflake safe space of a society, you might actually be able to get this teacher into a decent amount of hot water.
You should just buckle down and realize that life isn't easy and sometimes you have to work hard to get things done, I don't say this to be mean,
I say it for fear of what you'll be like in the real world after school and in the workforce. Your boss will not wait for you to get your shit done whenever you can focus on it, he wants it now or he'll fire you and hire someone who will do it on time.
Part Four: The Links
These are gonna help you.
https://www.understood.org/en/school-lea...-504-plans
https://www.additudemag.com/504-plan-ignored/
https://www.additudemag.com/dear-additud...ust-focus/
This one is just something I use to focus:
http://listenonrepeat.com/watch/?v=J9FIm...Theme_Song
If I need a laugh I like to listen to the above while viewing this gif:
http://i.4cdn.org/r9k/1493745043574.gif
Part Five: Forgot to say this
As far is a lawsuit goes, I don't think it would go well for you, if your situation was that serious you would have an IEP not a 504 plan, a 504 is for people who can't get an IEP, but still want special treatment for what they've been "diagnosed" with.