Dear Prussia,
When children are well behaved at home, and are not well behaved outside the home, it is generally regarded as a definite problem (the more normal situation relative to childhood behavior problems is for a child to behave well in out-of-home situations and to display some problems, not serious, at home).
You describe behavioral difficulties at school, and also mention that the school is telling you there is no problem with your son. What does this mean, do you think? What is the school trying to communcate to you?
Sometimes, human nature being what it is, we tend to become defensive if people suggest that there is something wrong with our own behavior. Ask the school what they think would be helpful, and express curiosity and interest in their point of view, even as it pertains to you.
At the very least, a systematic plan for behavior management will be needed, involving both positive reinforcement for your son's displaying the kinds of behavior you expect, and predictable consequences for not meeting those expectations. If you want help designing such a plan, consult with a pediatric behavioral health professional.
Children so young do not misbehave on purpose, so to speak. It's not as if there's a reason that is on their conscious minds. That is why parents often hear the words "I don't know" when they ask children why they misbehaved in some way. The challenge is to figure out a coherent framework for why your child is acting in this particular pattern. The behavioral health clinician can be of use to you in figuring this out as well.
Try specific praise for positive behavior and try ignoring the bad behavior. Yelling, spanking, and giving attention to the bad behavior usually makes it worse. Try giving him a reward for going a week at school with no outbursts, such as a trip to the park or a movie. My son works very well with motivators. He wanted the $36 Dinosaur egg so I told him if he could go a whole month with no anger problems at school we would get it - never dreaming he could go the whole month. He surprised us and the teacher by not only being nearly perfect but also helping to defuse problems with the other kids.
"The Defiant Child" is a good book on how to deal with this type of behavior.
Thanks again for the advise(confirmation).
Prussia
My 5 yr old is ADHD and it has taken MONTHS before he finally understood about rules, appropriate behavior, that yelling, aggressiveness and hitting is not the way to solve frustration and anger. So just be patient and keep going over the "rules" of life and remember to give specific praise for positive behavior to keep his self esteem high.