Plato believed education was essential for achieving justice in society. He proposed an educational system with two parts: elementary education from ages 5-20 focused on gymnastics and music, and higher education for the guardian class from ages 20-35 focused on mathematics, astronomy, and philosophy. Students would be tested at ages 20 and 30 to determine if they continued their education or entered other roles. Plato's educational philosophy emphasized developing both the mind and body through subjects like mathematics, poetry, and physical training to create a just and ideal state.