The Moon Incident

The story opens with Earth in disarray after 70% of the Moon is suddenly destroyed, leaving it permanently a crescent. Soon afterwards a yellow, tentacled creature claims responsibility and says he will destroy the Earth too unless he is killed before the end of the school year - meanwhile he will teach Class 3-E. Over the year the students learn the secrets behind the Moon's destruction, his link to their previous homeroom teacher, and the real reason he has to be killed.