Ages

BTS

Jimin-19 — senior
Jungkook-17 — (skipped a grade) Junior
V-19 — senior
Jin-22 — Youngest CEO in Seoul
RM-20 — college
J-Hope-20 — second dance teacher
Suga-21 — Basketball player

GOT7

Jackson-20, repeated a year — senior
Mark-21 — working at a company
Jin-Young-20 — college
Yugyeom-19 — senior
BamBam-17 — skipped a grade with Jungkook— junior
JB-20 — college
Young-Jae-20 — college

EXO

Chanyeol-19 — senior
Kai-22 — college
Sehun-20 — repeated a year with Baekhyun
Baekhyun-20 – repeated a year like Jackson
Lay-24 — music career
Suho-23 — Really young CEO
D.O.-22 — acting career
Xiumin-25 — music career
Chen-23 — university with Suho

EXTRAS

Mr.Lee-Lee Dong-hae-28 — dance teacher with J-Hope but with different classrooms.
Jennie-19 — senior
Yeri-17 — sophomore
Hyuna-22 — working at her brothers company.

Any other people in the story will be maybe mentioned later on.