What can i do if my son has been assaulted in school?

They--school and police--can do something; there is no law that says that an under-16-year old can assault another child w/impunity. You will need to escalate over the heads of the people who were not sympathetic; try talking to the principal, or, if necessary, a district level person--go all the way up to superintendent if you need to. Similarly, go over this officer's head and speak with a sergeant, a lieutenant, etc.

Although I do not practice law in the State of North Carolina, here are my initial impressions.  I suggest that you consult with a local attorney to discuss this matter and to then evaluate all options that you may have.  Many states have anti-bullying statutes, which would expose the school to civil liability in a situation like this.  Moreover, there may be criminal consequences for they children who are committing the assault.  After consulting with a local attorney and discussing this matter in greater detail, you will be able to identify the best solution to fix this problem.  Good luck.