Check out ODD as others have said. Consider a behavioral therapist? Schedule a conference w/ teachers, guidance counselor etc. For the most part they do very much care and want whats best for him. Look for support groups either in your area or online to ask other parents what they do. Be proactive! Keep a journal of his behavior. Did he not get enough sleep? Watch too much TV? Not enough outside time? Does he still eat grains? What is his sugar intake? Maybe there is a clue there you are missing. Ask school faculty the same thing. I know its hard when they are just kids to really monitor all of that but getting him off of it could make a difference. (you probably have answered this but I don't remember at the moment). While spanking may make you feel better it probably will not help. Good luck! I wish there were better answers.
You know all those things you wanted to do: You should go do them.
SW 215 lbs
CW 180 lbs (whole foods/primal eating)
LW 172 lbs
GW 125ish lbs