If you really want to fight back, fight smart and fight dirty, call the police immediately after anything illegal (violence and threats of violence are illegal)

If you really want to fight back, fight smart and fight dirty, call the police immediately after anything illegal (violence and threats of violence are illegal). File a report with the police after they arrive, photograph any bruises, etc. Upon repeat or grievous offenses, file appropriate lawsuits against the school and parents of bully. Yeah, it's lame, but not as lame as fighting fire with fire.