follow-up:

ok... Day three as a substitute and it went much better!

The morning class is full of cut-ups, But I told them that they had such and such assignment and joked with them about the letter the regular teacher left. ( about the ones that were specifically labeled as trouble) . I suggested a plan of attack for the assignment, and let them pair up if they kept the conversation to a minimum( the school requires silent classrooms-- Go figure how they're supposed to understand anything if they never talk about it, why not just hire the prison guards and be done with it?) I had only to remind them to be less loud two times in 90 minutes. No real hassles.
The next period was the BD kids that have remedial English. One kid refused to be reasonable, so he was escorted away. I feel really bad, because I later found paperwork that showed he had just got back from the alternative school. His behavior was effecting the rest, but I wish he could see his other options.
The next class was GT kids. I was a GT kid, My son's a GT kid. I get all of the nuances. I know the drawbacks, too. GT's need tools for effective learning just like ADHD kids do.They can get stuck in one spot.This class was as good as it gets. We connected. Absolutely no problems - work got done in about 40 minutes and we "socialized" quietly the rest of the class.
Now came the famous last class of the day: It was way better than the Monday was. There was one girl who wanted to start a fight, but she was talked out of it before it got out of hand.For this class I stole the "what are you doing?- what are you supposed to be doing..." thing from Covey's book. It works like a charm. they give you their best stupid grin and get back to work after minor grumbling. Great! I can live with minor grumbling. It lets them feel like working is their idea and like they wanted to do it anyway.. Also Great!

Not so great-- the last class didn't turn in much of the work. I guess they need some more time to finish? Or they are are just tired of the BS assignments by the end of the day. I've seen the work they are given. It's like toast compared to red velvet cake. Yes, you'll survive on it, but it isn't any fun to eat.