Styles have varied iver time and from one school to another. Most private schools required uniforms. Chronology Argentine school children commonly wear smocks. I'm not sure when that first begun, but continues into the 2000s. Other than the smock, state schools do not require uniforms.
Primary children wear what ever they want under their smocks.
Secondary children dress as they wish. Most private schools do require uniforms. Suits were common in the 1950s, often short pants suits worn with kneesocks. Primary children at private schools wear smocks like at the state schools, although they may be colored. Many private schools are run by the Catholic Church.