How are teachers supposed to deal with physical violence from pupils?

I am a newly qualified teacher and my lecturer would not answer this question saying that it would never happen? But how is he to know! What I want to know is the legislation in place or the protocol for teachers if a pupil is physically violent towards them. I know we cannot use physical force on a pupil but if they are posing a threat and hitting or throwing punches, surely we are allowed to do more than shield ourselves? I ask this because my sister is a nurse and has recently been the victim of a serious ault by a patient as she wasn't allowed to restrain the patient?