given the flashbacks, it seemed like he did specifically wanna relive the high school since it was so awful for him before!! he also gets his GED later so it appears he didn’t actually finish highschool in the first place.
it also seemed to me the way baek explained everything was that to the other kids, he would appear younger so they wouldn’t question his age or where he came from (like previous school type information)
certainly could be wrong but thats how i took it as i was reading

So is he a grown adult at least 20?!? Cause why tf he end up in highschool when he said he wanted to go back to school? unless that’s specifically what he wanted to go back to, maybe I missed something