Have you talked to your doctor about this? I think some counseling might be beneficial. Has he always been like this or is this a sudden change.
Is there any kind of violence in his life he may be copying what does he watch on TV as his Dad do you put movies on that he would see that have violence in them or anyone he visits , family ? Something is upsetting him, has he any older siblings, hitting a teacher is usually a sign that he does need some help maybe some counselling ask your Doctor first what he thinks is causing this behavior. Is he on any Meds?