One possible theme could be the importance of education. Since it's a teacher's story, it might focus on how education can change lives.
I'm not sure as I haven't read it specifically. But generally, it might be about a teacher's experiences, perhaps in the classroom, with students, or about the teacher's own journey in the teaching profession.
Another theme could be personal growth. The English teacher may experience growth in their own skills or understanding as they interact with students. For example, they might learn about different cultures from their students and this broadens their perspective. Also, students in the story could experience personal growth through the influence of the English teacher. They might gain confidence in speaking English or develop a love for literature.
Another theme might be about authority. Since it involves a teacher, it could explore how the teacher exerts or loses their authority in the situation described in the 'facesit story'. For example, if the teacher faces a mutiny of sorts from the students, how does the teacher deal with it to maintain their authority? Also, the theme of growth could be there. The teacher might learn and grow from the experience they face in this story.
One possible theme could be growth. Maybe Chase grew as a person or a student because of his teacher's influence.
Existentialism could be a theme. As the end approaches, characters may question their purpose and existence in a world that is about to cease. They might struggle with the meaning of their lives and their actions in such a dire situation.
One possible theme could be friendship. The relationship between the teacher and the 'teacher's pet' could be seen as a special kind of friendship that develops during their shared 'journeys'.
One possible theme could be dedication. The teacher's obsession might stem from a deep dedication to their students' education, like ensuring they all pass a crucial exam or get into good colleges.
One possible theme could be growth. As the teacher guides the student to go deeper, the student might experience personal and academic growth.
One possible theme could be dedication. Teachers are often dedicated to their students' learning, and this might be shown in the story.
One possible theme could be adaptation. Teachers in 2020 had to adapt to new teaching technologies and formats.