Arguably the worst three years of any child's schooling. You feel like you're too old to be treated like a young child but you're treated like one anyway. Some students test the limits, and as such the entire student body is restricted even more. You slowly loose your freedoms one by one until you feel like you might as well be in preschool for all the independence you have.
Also, it is close to guaranteed that in at least one of your middle school years, close to all of your friends will dump you. You know back in fifth grade when you and your BFF 4 LYFE swore you'd stay friends all the way through FOREVER? Yeah. That pretty much all goes to shit the second you hit middle school. People grow apart or become popular wannabes or just move on to different people. It sucks. You'll make new friends though. Usually.
Organization is a HUGE deal in Middle School. Teachers are all over you all the time, making sure you have every binder, paper, and book in place. It's obnoxoius and invasive. You'll get used to it.
Fifth Grade Girl: OMG! I can't wait to go to middle school! I'm going to be all grown up and do whatever I want and my friends and I will be popular and all the boys will want to date us.
8th Graders: (trying to contain hysterical laughter) Mmhmm. Sure. Whatever you say.