At six, saying things like "I'm concerned..." and actually be concerned, not sarcastic.
Kids always fall for the "I'm concerned" line at this age.
Then start a dialogue about sticking his tongue out, or refusing to do the work. Ask him if the other kids are sticking their tongue out at the teacher. If he says no, ask him if he feels bad about himself when he does this. *I had some behavior problems in school, and while I didn't know how to think first, act later, I ALWAYS felt really bad about myself. I still look back on the things I did with humiliation. It would have helped if someone had talked to me about how I made myself feel about ME at that age*
It's no fun to be "that" kid. It just isn't. Punishments don't help (I know you never mentioned punishments) It feels like a cycle that you can't get yourself out of. Once you have a reputation (real or not) you just don't know how to stop it and reinvent yourself. You feel like you have to keep being that kid because that is what others expect of you.
Help him with ideas on what he thinks might help...because he doesn't deserve to always be that kid. It's great to be the free thinker, and have ideas of your own...but, it's not O.K to be disrespectful, just for the sake of being disrespectful. Besides that, he's older than the Tongue behavior. That's so three year old. He can find a better way to speak his own mind without being ugly to others, and people will want to listen to him if he's discovered a better way to make his point.
Help him learn tools for this. He deserves so much more than what he's creating for himself. He deserves to finish his work and be outside playing without that "i'm not done" feeling hanging over him. He deserves to feel proud of his work, and the freedom that only comes with knowing it's done.
You know how you feel at tax time? Then how you feel when it's done and been mailed off? That's what he deserves for himself with 1st grade work.
Because I hated being the kid who never finished her work, or the kid who mouthed off in front of the other kids. I hated knowing the other kids in the class went home and said "Guess what K did today?"