They can call it a "Dress Code" all they want, but it is a uniform.
The definition of a school uniform is: Standardized clothing for regular school attendance worn in conformity with school policy. or — n 1.a prescribed identifying set of clothes for the members of an organization, such as soldiers or schoolchildren 2.a single set of such clothes 3.a characteristic feature or fashion of some class or group 4.informal a police officer who wears a uniform