Status Updates From Assassination Classroom 1

It is about a boy named Nagisa who is a junior in high school in class E, which is a class for the not-so-good people. One random day a creature that looks like an octopus, who before disintegrated half of the moon, goes into the classroom to teach. A secret agency goes in and tells the kids that the octopus isn't able to be killed easily because he moves 20 mach(very fast), not even a missile can kill him.