don't take it personally. each room has its specialized staff with a certain job description and you shouldn't feel slighted, it would rather be unfair for your child to be put in a room with others that are already potty trained, rules are rules, after all. besides, i'm sure you wouldn't want your child to be the *only one* not potty trained in the entire classroom and have to hear comments from other kids because "he's stinky" or still wears a diaper, etc. you don't want him to be embarassed every time he needs to be taken for a change. if he really wants to be in the other class this could encourage him with the potty training. (i have a son with autism (now 18) so i can say this without any bias and understanding how you feel, i've learned sometimes you need to do what is asked of you, and other times you need to fight for things. i would consider what it is you are fighting for right now, the benefits may not be as ample as you think) it is important for EVERY child to feel succesful and proud of himself, and you wouldn't want him to feel less than the other kids.
have a great day~