You should've gone to your parent(s) from the get-go. No, it's not childish. It takes a parent to butt-f&#k the principal in his/her own office with every legal threat to get anything done nowadays. What happened to you is/was unfair, no, you don't have to take it, but no, don't expect to get any understanding from the "adults" there. Go to your family, plead your case, go in with them prior to start of class, and sit in the hallway as they put on the long-armed latex gloves. Then, once the principal has conceded, make eye contact with them and any teacher you can afterwards. Show them you can NOT be intimidated, you won't stand for their Communistic crap, and proceed to class as normal.
I took on an entire school when they tried to pull shit like that on me. I was attacked by the school bully at the time on school grounds, and BOTH of us were scheduled for ISS (I tried to plead my case on deaf ears as well). My family tried to argue with them about why I was going in as well, they wouldn't listen to them either. On the day my ISS was to start, dad went into the office WITH 2 police officers in tow to have the bastard AND the teacher arrested for assault since he stood by while I was being wailed on (I never even threw a punch, he had suckered me before I knew he was there...), and the principal received a call five minutes later from my dad's attorney ready to file a lawsuit on the school. After he collapsed on his ego like a scared puppy, he cleared me on all demerits earned for the ISS (and the rest of the year), called the bully's parents to be there when the police took their kid into custody, and followed my dad out the front of the school doors begging not to be sued, just as the newspaper interviewers had arrived (that lawyer had INCREDIBLE timing!!!).
Legal threats go a LOT farther than logic to a school system, trust me.