My 5 year old will not behave at school, what do I do?

He is an only child at the moment & is attending kindergarten at a public school. At home he loves helping with chores, helping to pack his lunch, offering to help clean, sharing with friends at play dates, etc. But when he is at school he has a lack of self control. He does not want to participate in group learning, he talks back to the teacher, hits/throws items at the other students, refuses to apologize, etc. To know how well behaved my son is at home & then to hear how awful he behaves at school, is just heart breaking. He knows better, he was not raised to act that way nor treat others in that type of way. Discussing his behavior, going over the correct way to behave at school, & the proper way to treat classmates, has just not seemed to help. How should I handle this?