I work at a school. In all of the cases that I have personally seen, when the child is acting out in the classroom he or she has a dysfunctional home life (e.g., fighting parents, alcoholic parents, parents who refuse to send the child anything for lunch but a moldy piece of bread, etc.).
I also work at an art gallery/custom framing store. I had one mother bring in her two boys, about 5 and 6 and they obviously had issues. The second they walked it she let them loose, running around glass shelves, chasing each other, screaming, jumping, shoving each other...she just blatantly ignored them and talked to my boss and I as if there were not two terrors about to destroy our store in the background. It was very awkward....I didn't have the courage to say, "Dude, lady. Restrain your children before they knock something off the wall and bust their heads open." I later found out that those kids used to go to the school I now work at, but got kicked out within the first year.
Not to say your child has a dysfunctional home life, I'm just saying it has been observed in 100% of the cases I have seen.