Thought they're in highschool ..
It started with their highschool years in the 4koma gag chapters, then when it starts to get more serious with an actual plot, it's all a flashback. Early on, Zheng-Xi said that Yi disappeared the 2ed day of highschool, but they're still in their Jr High uniforms, so I think he's going to get kidnapped by his father's men again.
They're currently in middle school. Jian disappeared their second day of high school. Everything you're seeing right now is a flasback from the time before high school, so they're in middle school.
Sometimes I forget these kids are supposed to be in middle school and are like 14/15 when they've got bodies like that