Overall it was beautiful anime with some nice scenes and stories, but as a whole anime the pacing was way off. In the end, nothing about the world, war or why Mimi is immortal is revealed and concluded. I'm not sure if the story is similar in the manga, but for this single season nothing was wrapped up or explained. The characters don't have enough backstories at all and the whole school and world still feels like a mystery. The part where all the main and side characters are only female is weird also since the school actually has male students and teachers that appear occasionally.