I would immediately report the incident either to the Principal or to the Deputy Principal, and would document the event. In addition, if the situation is serious and threatening to health, I would restrain the child and I would ask for the security officer at the school to escort the child out of the classroom. I would also contact the parents.
I would actually try to protect the children involved. Hitting a student is not acceptable and such a case needs to be dealt with quickly.
To answer this question properly, you can also talk about your discipline philosophy...