As far as the bathroom-aid thing goes, I have NEVER had one. Heck, in the school I went to for grade 6, you did not even ask to go to the bathroom (it was a 5/6 split class). There was a pass for the girl bathroom and one for the boy bathroom, if you had to go, you quietly got up, grabbed the pass, put it on your desk, and went to the bathroom. That way, the teacher could see the pass and know you were in the bathroom if they were looking for you. And that way only one student of each gender was in the bathroom at one time, unless the other student was sent in because you were taking too long in the bathroom. You also grabbed those passes if you had to run to your locker or anything. Really, I guess it was more of a "leave the classroom" pass than a bathroom pass.
Becca, I understand being upset about it. I would be. A teacher tried that with me in grades 4 and 5, and my dad was PISSED. I actually never ended up staying at school for more than half a day those 2 years, because if the teacher was trying that I was instructed to just get up and leave the school. (NOT saying that is what you should do, just saying what I did in similar circumstances). Of course, in my cause it WAS dangerous for me to try and fight it with the teacher (the teacher physically grabbed me and fought me if I tried to go to the bathroom).
I would, as was suggested, make a meeting. If the meeting does not work, go the next step higher than the principal. If you do that, and the teacher and/or principal retaliates on your son, which it sounds like your son WOULD tell you, keep bringing it up with the school board or the principal or, if need be, bring it up with the police. Honestly, one call to the board and letting them know that you are aware what they are doing is illegal, and are not afraid to take it up to the next step, will stop the issue.
I would also demand that the teacher no longer teaches your son.