behavior problem. Please help me!!!

I have two boys. My oldest 8 years old boy had several behavior problems at a public school. He pushed, threw sand to other kids, said "You are not my friend. I will kill you", "I will dig a hole and bury you in the sand, and then you can’t go anywhere.” The teachers really concerned and want us to take him to see social worker/psychologist. He is a good kid and always gets A in class. Both of his piano teacher and Chinese school teacher said he is a sweet and polite boy. I feel he is not mature comparing to his age kid. He thinks everything is funny and hardly gets serious of something. We spanked him sometimes. I don’t know how the system in US works and I don’t want to ruin his future. Please help me and give me some suggestions.