There may be fanfictions that explore this idea in a family - friendly manner. Maybe it's set in an alternate universe where the 'pregnancy' is a result of some unique magical or scientific experiment that has nothing to do with adult situations. Writers often use such concepts to explore different aspects of a character's personality and how they would react in unexpected situations.
In a fanfic, their relationship might develop in a more romantic way. They could be paired up for a school dance or some other event. During this time, they have a chance to really talk and get to know each other on a more personal level. Nagisa might be shy at first, but Kayano's friendly nature breaks the ice. As they dance or spend time together, they feel a spark. This could be the start of a romantic relationship, with them stealing glances at each other in class and finding excuses to be around each other more often.
One possible plot could be a story about Nagisa and Kayano working together on a special project for the class. They might be assigned to plan a surprise event for Koro - sensei. Along the way, they discover new things about each other. Nagisa could learn more about Kayano's acting skills and how she uses them not just on stage but also in her daily interactions. Kayano could see Nagisa's determination and kindness up close, which deepens their friendship.
One possible storyline could be a story of misdiagnosis. Kayano might be wrongly thought to be pregnant due to some strange alien or experimental side - effect in the Assassination Classroom world. This would lead to comical misunderstandings among her classmates and teacher, and the story could revolve around clearing up the confusion while also exploring her relationships with others.
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.