
I immediately clocked the age difference and honestly the fact that she was his teacher was more of an issue than the age. I think its suppose to set up a dynamic between her work related romance options. I dont want to judge a book by its cover, but tropes do exist. I wont say the coworker is a stalker(ugly bastard trope), and i will gibe him the benifit of the doubt. He obviously looks at her social media so ultimately his reaction to her being with her student(hes gonna cross paths its simple writing) if he gets angry hes a bad guy, if he gives her space hes good. Ultimately its up to her to repsect her own boundaries (like dating a dtudent) and him to acknowledge them

The truth will come out eventually, but really keep an extremely close eye on his reactions. If he knows more than normal about her activities, or acts like hes entitled to date her /gets upset and makes assumptions, drop him and just accept the student is her love interest.
If he passes the tests with flying colors, then shes gonna end up with a situationship at best wjth the student and crawl back to him realizing hes everything she wanted in a guy, but like the guys she slept with it was only a surface level thing she kept pursuing. By that point she either loses the bet as a single woman, or dates the coworker.
If he does crash out just know her job is at risk
I'm a person who finds age gaps very icky and a big turn off so i take this matter seriously and did the math. This is just my assumption but going by the facts we alrady know:
Blondie is a 31yo highschool art teacher and the
ex student is now in college as it's revealed in coversation in chap 9. Then i chap 10, he asks her if 6 years ago she worked in jinja highschool
Hyun bin was in her 3rd grade class. I think it could have been translated better, but it means he was a senior in highschool (as in Korea highschool is only 3 years).
Also going by the fact that she is in denial and thinks he is only interested in her bcz he supossedly wants to ask her about the teaching exam preparations, that could mean he is almost about to graduate college. But this last bit could be a stretch
So doing the math, present day he should be around 24yo. As they met when he was 18/19 (korean age) yo and 6 years have gone by.. Meanwhile she is 31 yo
So...
They have a 6/7ish age gap
Fortunately I don't feel icky about their age gap and I'm praying I guessed their ages right, and he isn't freshly out of highschool.
Still he is the young and impressionable one. But i feel that their relationship will be healthy and give space for a lot of growth for both of them as they are still newbies to relationships.
A pure hearted younger guy maybe was the type of guy Seon needed