Mostly, but there might be some minor details or side stories in the manga that weren't included in the anime. However, the core plot and major events are covered.
The 'Assassination Classroom' is about a powerful octopus - like creature that destroys a large part of the moon and threatens to do the same to Earth. He becomes a teacher of a class of misfit students. The students are tasked with assassinating him within a year. As the story progresses, the students grow both in their assassination skills and as individuals. They form bonds with their strange teacher, who also teaches them valuable academic and life lessons.
One of the main science fiction elements in 'Assassination Classroom' is the character of Koro - sensei. He is an octopus - like being with extraordinary powers, which is a very imaginative and science - fictional concept.