The word "school" has a long and interesting etymology. It ultimately comes from the Ancient Greek word σχολή (skholē), which originally meant "leisure" or "free time." This is because, in ancient Greece, schools were places where free citizens had leisure time to pursue learning and intellectual pursuits, as opposed to working.
Over time, the meaning evolved. From "leisure" it shifted to mean a "place of leisure" specifically dedicated to learning, and eventually simply to the institution itself – the "school." This evolution in meaning occurred over centuries as the concept of education and its institutions developed and spread throughout Europe.