Assassination Classroom OVA

The story of Assassination Classroom revolves around a middle school class and its teacher, which happens to be an alien. The alien is a high speed, tentacle monster that was known to destroy 70% of the moon. The alien makes a bet with humanity. If he is assassinated before graduation, the Earth is spared. However, if he is still alive by then, humanity is done for. It is the duty of the students in his class to learn the ways of the assassin in order to kill their Alien teacher (it also helps that there’s a 10 billion yen reward for whoever kills him).