How Does A Parent Go About Making A Complaint About Their Child’s Head Teacher?

My husband and I have this morning been to see our son’s headteacher about an incident that happened where he got out of the school gate and into the street. The headteacher said that they are not allowed to lock the gate, it is only bolted with a slide bolt and no padlock. Is it going to take the death of a child before they step up the security? Any school governors out there for some good advice please.