
It's a bit vague atm, since the comic itself hasn't mentioned any ages or what he's studying for, so we only know that he's studying at some academy (no mention of whether it's HS or cram school), hence the confusion.
Tho, I have no idea if Koreans study for their CSAT during HS, or after just graduating. Bc where I live, we do our college entrance exams during HS.

I've seen several manhwas with characters that have graduated high school, but are studying for CSAT for a variety of reasons. The most common reason they usually give is that they didn't do well enough to get into the university they wanted. I've also seen academy used as shorthand for cram school like a learning academy.
In addition, I looked up the webnovel summary. The MC is described as a third time repeater meaning he's taking the exam for the third time
this is so childish its actually funny lmfao.
Do I cringe at whitejack? yes. Is this a cliché story? yes. Am I still gonna read this cliché shit? also yes because I'm a basic bitch who loves cliché(and also because it's good enough for passing time)