Wow: have you had good luck. I am a former English teacher now a lawyer. "Negligence" is a concept of Anglo jurisprudence. It refers to a person who does not do his legal duty, and may be held responsible at law for injuries to a person in his care. If a headmaster is not responsible for an act (let us say injury to a student) "unless negligent", it means that he would not be liable at law if he could show that he acted properly within the duties of his job.