Legal Help Needed

My son, a 6th grader, was playing with his friend at school. He pushed his friend and boy then bumpped into a girl next to him. The girl hit a locker (metal) on her head. At the time, my son appligized and the girl felt fine. The next day, the girl had a headache and went to ER. The girl had a CT scan. The doctor said that she had a mild concussion.
The girl's family asked school for accident insurance. The school asked us to contact the family to discuss the medical cost.
We don't know how much the medical cost is yet. What are my options?
Is our family liable for the medical cost? Has anyone had this experience before?
Thanks.

Jean