Answer: The Aztec Empire
Although many cultures had educational systems in place before the Aztec Empire rose to power in the 14th century, the Aztecs introduced something previously unheard of anywhere in the world: universal compulsory education for children of all genders and ages.
While other cultures prior to the Aztecs had elements resembling the concept—in ancient Sparta, boys were put into a strict military training system and in ancient Judea, boys were required to attend school—no country or empire had sent all their children, regardless of gender, to school in such a fashion.
Younger children were instructed under the tutelage of their parents, but supervised and tested by local education authorities. During their teenage years, both male and female children were required to engage in formal study at either the Telpochcalli (which focused on the study of history, religion, military fighting arts, and trades/crafts) or the Calmecac (which focused on advanced learning in writing, astronomy, statesmanship, theology, scholarship/teaching, healing, codex painting, and, as at the Telpochcalli, military arts.).
It would be centuries after the fall of the Aztec Empire in the 16th century before any other political states or countries would adopt universal compulsory education on a border-to-border level. The modern universal compulsory education system familiar to students (and graduates) the world over was developed in Prussia in the 18th century and had spread to most Western nations, including the United States, by the 19th century.