I am really glad to hear that you are going to cut down on his punishments. Kudos for all of the above for convincing you to do so. Let me also add that for a child of this age, to punish them for something that they did hours earlier is a waste of time. They don't have the capability to connect the two things. By the age of maybe 9 or 10, yes - at 6 its developmentally extremely difficult.
A couple more suggestions. There are several really good sets of books that are meant to be read to the 4 to 7 year old child. I'm not exactly sure which book would be the most helpful, but "know and follow rules" might be a good start. It can be found here - http://www.amazon.com/Follow-Rules-Cheri-Meiners-M-Ed/dp/1575421305/ref=pd_sim_b_4
And if you scroll down, you can find many other books that will be helpful. One of the things that these books do (besides a neat bonding time as you read to him) is to give both of you a common way to discuss things.
The other thought I have (which is from monitoring the ADHD forum for many years) is that it is always possible that he really can't control what he is doing due to ADHD. If it is something like ADHD, then it really changes how you deal with him, his teacher, and really everything. So its probably worth your time to check out the possibility. A good starting site is -
If any of this seems possible then I highly recommend you get the book, "The ADD/ ADHD Answer book," by Susan Ashley. It will help you deal with the school now and in the future. But mainly, it will be a resource for you for many years to come.
I hope this helps. If it does look like ADHD, I monitor the ADHD forum found here - http://www.medhelp.org/forums/ADD---ADHD/show/175
Its pretty easy to find me and my answers to posts, so feel free to also post there. I will monitor both sites. Best wishes.