Assassination Classroom

The story of an alien creature - kind of a yellow smiley face welded onto an octopus like body - that has destroyed most of the moon before arriving on Earth and informing humanity that it will do the same to our planet within the next year and, oh yes, it'd like to be a high school teacher while waiting to do so ... well, nobody is likely to accuse Matsui of stealing their ideas. Particularly not the bit about the alien teacher (Korosensei) spending its days teaching its new pupils assassination techniques to try and kill it before it has the chance of destroying the planet.