Read chapter 45, it explains that they were classmates through-out Middle School, but on the 2nd day of High School Jian disappears and he doesn't return until after Xian (spiky hair) has already graduated from HS and is a college student.
Because Jian was gone for years, he has to re-enroll in HS even though he is an adult. Also, in many chapters before 45, Jian is shown drinking alcohol even though he is wearing a school uniform (because although he is a student, he is an adult) and we NEVER see Xian wearing a uniform along with him (he's in college=no uniform)

So if they are in thier last year of middle school and they are all around 15 and the flashback ends when they are 18 that means he tian will be gone for 3 years oof ╥---╥