Sometimes you’ve got to be more adult than the adults. But since the adults have the power, you need to be very, very cautious.
As said before the 504 plan is a legal document. Read it, learn it, memorize it, carry a copy with you, know it backwards and forwards. Be prepared to recite the pertinent parts if necessary.
Do not fight the coaches or teachers on their turf - in the gym or the classroom. You cannot win. You can state your position and the requirements of the 504 plan, but unless they demand you do something unsafe, do what they say and do it without a scene.
But document everything. Dates, times, conversations, who was there. Don’t give your documents to anyone. Give them copies.
Make your complaints to the administrators, and don’t be smug if they take your side. Be a good sport about it, shake the teacher/coach’s hand (no matter how big an ass they may be) thank them for participating and tell them how much you look forward to being in their class. Do NOT gloat or brag about getting your way over a teacher or coach - in other words, don’t rub their faces in it.
I’m serious. Be more adult than the adults. Even though you may have every right to be ticked off and scream ‘unfair’, it has not affect for one reason - teenagers are always ticked off and think everything is unfair as far as adults are concerned. So when things really ARE unfair - it’s hard for the adults to see it. In addition, keep in mind that things often really are unfair even for adults. Many adults take the attitude that “I have to deal with unfair crap all the time, so what are YOU complaining about?”
I don’t mean for this to come across as a lecture - more as a field guide to adults.
Emotional responses lead to emotional results. Practical responses get practical results.