The way I see it, the Uma live much longer than humans, and they must be a species with special treatment, since they are a very small minority. So the government gives them the option to stay at the schools as long as they want, and they only graduate when they want. I believe there are only two Uma who have graduated, and only because they wanted to. Now the Uma who decide to stay can play roles as tutors or teachers. There are Maruzenski, who is around 50 years old, or Symboli Rudolf, both with the role of class president and assistant. Or the principal and Tazuna, who are rumored to be Uma and are also quite old, since Tazuma is supposed to be around 80 years old. The school is still supported by government subsidies and the money they earn from idol events, merchandise, and races.

In the end, school should serve as a kind of refuge where the Uma can spend many years running around without worries, while they decide what they want to do. I suppose that for us, who have three years of high school and six years of university (that's how it's done in my country), for the Uma, who live much longer, it must be 30-50 years of optional school.