It is a bit of stretch to say that Plato's Republic-(and Plato in general), established (or even popularized), the concept or policy of a mass educational system-(similar to what exists in much of the world today). Plato's Academy, was very much, a clubby, members only type of institution with its famed inscription-("Only Mathematicians can enter"...I am paraphrasing the statement and I apologize for its inexactness). Plato's Academy was an educational institution, but it also functioned as a fraternal organization, a type of intellectual brotherhood-(as one of the Commentators stated). The design and purpose of Plato's Academy was actually, the opposite of a mass educational system.
It is difficult to find examples of a mass educational system in the Ancient world, with the possible exception of Alexandria, Egypt during its Hellenistic and Roman heydays. Ancient Alexandria was home to the world's largest and most cosmopolitan Library, though it was also home to the world's 1st College/University-(which is lesser known). But, even the Alexandrian Library/College was still, quite elitist and only available to the (more well-to-do) citizens of The Ptolemaic and later Roman Empires.
Comparatively speaking, the Alexandrian Library/College would have been viewed as a Proletariat's Paradise by Plato and the first generation of Academy students.