Pupils who have done nothing wrong to merit expulsion are sometimes expelled if the school does not expect them to achieve sufficiently high grades in external examinations. This illegal policy seriously harms the life chances of young people. Since many violent students often rebel against school rules, some head teachers may choose to expel a student who has performed an act of violence against another pupil for "persistent defiance" rather than violence. This is in order to protect the victim from being assaulted again as revenge for the excluded student's expulsion. Some regard this totting up process of persistent defiance to be unfair, as the pupil will have often been punished once already for each act, and expulsion can be seen as a second punishment imposed after the matter had been settled.