The pupil's name and details of the assault would be published on the school website and circulated to any regional forums in order to cause maximum shame. The local press would be contacted with the story and to hell with any concerns about the school's image. (I know full well that some wretches might regard this as a 'badge of honour', but possible future employers won't). No future member of the family would be accepted into the school.
If the criminal prosecution proved to be unsuccessful then the school would immediately instigate civil proceedings.
The only reason all schools don't do these things already is that the Heads aren't up to it.