Animation.?
Excellent question fam
It depends on which time of the story you're talking about
In the beginning of the webtoon, they are all in high school and then it shifts over to a looooooooong ass flashback of Seungtaek and the green/brown-haired guy (ugh i forgot his name) and how they met, which was in middle school. Then it goes back to high school I think?
Since the animation strictly focuses on Seungtaek and his partner and completely leaves Seunghee out of the story, it takes place during the flashback in middle school with Seungtaek in focus
does anyone know how old they are. while reading the raws i was under the impression they were in highschool but when i watched the animation, seungtaek said he was in his third year of middle school which in korea would be around 14 years old right?