Plato's desire was to make an ideal state or just state. A state where citizen's life can enrich and grow virtuously. In the words of Plato, State is individual writ large. In a sense, Plato's conception of justice of citizens is related to the just state. In his theory of education, Plato believes education is a way to find the aptitude of a person. According to Plato, the goal is to go beyond senses (world of matter) to pure conceptual knowledge (world of idea). True knowledge guide to take the right decision. In the human soul, three spiritual qualities exist. Those are the reason, courage and appetite. And one thing dominates out of three which defines human character, Plato believes with his method of education which is capable in finding the true quality of soul, a man in which appetite dominates will become the part of producing class, a man of courage become part of a soldier class and a reasoned man become a member of the ruling class.
Plato gives a holistic view of education. Primary education(0-6 years) character building, Secondary education (6-18 years) basic knowledge of different subjects with a first screening test, those who pass will study further and failed will join the producing class, (18-20 years) compulsory military education with a second screening test, passed will study and rest join soldier class and finally higher education(20-50 years) they will be part of ruling class and only one in them will become philosopher king.
According to Plato's theory of justice, a just society is in which people have virtues, follow their duty/obligation toward the state because these things will bring peace, harmony and excellence. Justice will come if we can find the just soul, a soul of reason because only the just soul of a just man can bring justice. According to Plato, with two conditions justice can be maintained. If there is a functional specialization means all three classes ruling, soldier and producing do their work with sincerity and the second is non-interference into each other work.