Five-year-old Kai is a transgender girl beginning kindergarten in two weeks - and the school board won't let her use the female washroom.

The lawsuit asks a North Texas federal court to declare the directive unlawful in what ranks among the most co-ordinated and visible legal challenges by states over the socially divisive issue of bathroom rights for transgender persons.

A potentially epic clash over transgender rights took shape Monday when the U.S. Justice Department sued North Carolina over the state's new bathroom law.