2 things probably, early education especially requires a looot more qualifications than any other (I know college professors younger than kindergarten teachers), and it seems he did graduate right before the apocalypse because he was celebrating in some way when his father dies
Some people take a break after finishing high/ secondary school. or maybe he was studying sth else before and then went on to study to become a teacher.
I'm not sure how it works in Korea but college isn't usually strict with ages. Like you could be 40 and apply for a college course
Maybe he did his military service as well
I’ll assume he had his own personal circunstances like taking care of his mother and the apocalypse started right after graduation.
The apocalypse has be going on for at least 2 years because the girl Gyeongha met Guwon when she was in highschool and she is currently 22. Has the teacher never killed anyone during all these years? The rest admited they have killed people outside. Hehe I guess we’ll learn in the next season.
Teach is 29 years old and he never worked as a teacher. What have have you done all this time? Or the apocalypsis started right away after graduation?