6 year old boy behavior problems

My oldest son is 6 years old (birthday in July).  He completed Kindergarten last May and had a good year for the most part.  He occasionally got on "red", but rarely.  Over the summer we moved to the town where his grandparents live.  He had a rough first part of the summer dealing with moving and leaving his neighbor friends, etc., (throwing crazy fits, losing his temper, easily brought to tears - distressed) but seemed to be really improving as the summer went on.  He began first grade at his new school in August and has really been struggling with behavior in class.  We were pleased at first to learn that his best buddy down the street is in his class, but when we saw the two were assigned to sit next to one another at the open house, we quickly informed the teacher that would not be wise so she moved their seats apart before classes began.  His is typically getting comments/notes home for talking during class, not listening, not following directions.  He rarely stays on "green" which is frustrating to him.  Our policy is to mark any greens on a paper on the fridge and when he has collected 10 he gets a reward that he chooses (within reason).  If he comes home with yellow we tell him that yellow is okay.  If it's orange we just encourage him by telling he can try again the next day.  If he comes home with red we usually discuss what he was in trouble for and if it is something rather severe he is not allowed to play with his friends that evening.  He was recently sent to the office which was very upsetting to us.  We had a stern discussion with him about it and he was grounded for one week.  I worry that school is becoming such a negative experience for him that he'll never learn to enjoy it and that he will continue to behave badly b/c eventually that's what people will expect him to do.  His teacher mentioned he may just be immature and we may just have to wade the water until he grows out of it, but in the mean time it is very distressing to receive negative comments about your child on a daily basis which I'm sure is coming through and causing this frustration at home.  He also is sometimes proned to "poor me" mentality...no one likes me, I don't have any friends, everyone hates me, I'm a bad kid, etc.  He did that some even last year, but it has gotten worse this year.  He is not a bad kid and he isn't always in trouble, but in our effort to try to help him understand how important his behavior at school is and expressing our frustration I feel is just making things worse.  I'm sure he is feeling attacked from every angle.  He is very sensitive which sometimes causes things to be blown out of proportion (I'm a little that way myself so I can sympathize.) We are frustrated as parents at his behavior, but we are out of ideas to help him improve and feel happy and secure with himself.  
Any advice on how we can help him with his behavior and social/emotional distress?  Or, how to deal with his teacher/school would be greatly appreciated.