That’s because a lot of the children not in school are busy training for war for about five hours of the day. The world is an eternal night, no sun but two moons. There are ancient artificial lights that reflect the moonlight so the planet can see, but some are broken, so about five hours of the day it’s pitch black, so it’s hard for the people that can’t see completely in the dark, so those are the hours completely off-job (apart from guarding outposts and stuff). So, ten hours of school, five hours of training, making about fifteen hours of school, then there’s thirty five hours off, including the five lightless hours, which is curfew, and the thirty free hours is usually used for jobs to help the kingdom or more school. And the school has a lot of homework.