You could also reach out to the other parents and ask if they know what's going on. I don't mean that you should show up at their door ready to rumble. Just tell them what your child told you, acknowledge that you aren't there and might not know the whole story, and ask if their child has said anything. Odds are that they will be more chagrined than anything else.
Bullies often act out because of issues at home. Several people I know tried to discuss things with the other parent with the following results:
Were told their kid is too sensitive
Were told their kid is making things up
Were told their kid is the one doing the bullying
Were thanked for the information - other parent had a talk with their child who then took the bullying game up to the next level
It's best to work with the teacher/school counselor to find ways your child can manage the situation. The school can help by separating them as much as possible. It needs to be collaborative - you, your child, and the school working to make things better. Do not expect the bully to change his/her behavior or for the school to move quickly. Since there has been physical bullying in your child's case, you should push to have them separated. Your child's personal safety while on school grounds is absolutely their responsibility - be sure they are aware of the physical nature of the bullying and that it happened at school and find out what their plan is to address it.