Ds is 11 and in year 6. He has many good friends and they've always had a bit of healthy banter about football.
Today on the short walk home from school he and his friend were arguing, again over football. It appears it got a bit heated.
The friend was calling ds names, ds told him to fuck off (I have made thing clear to ds that this is unacceptable)
The friend then hit ds in the face 3 times busting his lip. Ds also is complaining of a sore jaw. He is a little shaken and upset too. 
I'm livid but I don't know where this child lives and previous conversations with the mother lead me to think she would react to me with anger.
I know it was outside of school but I'm thinking I could phone school and ask ds's teacher, who is also deputy head, to speak to this boy tomorrow about how his behaviour was unacceptable.
I'm just not sure I'm doing the right thing. Help!