When you're being bullied or in a difficult situation, it's hard to tell people, no matter if you trust them or not. This might not be the case for Yeonwoo but based on previous chapters, I'm thinking some of the reason he might not have told them is because he wants to stop relying on others to fix his issues, that's a guess though.
Anyway, the point stands that telling people is gonna be hard especially when you're scared of the person (or people) hurting you.
I agree with you. For a child a to stop relying on their parents somewhere its broken. When you are in need you usually call on your best ally, but he didn't choose any of them to consult. Which means he didn't believe he have any true allies. He still could have done a joint front and talk with his friend in his class that stood up to defend him.Maybe they could have come up with another solution.
I want to know why didn't he talk to his parent they look like nice people that will listen to him and protect him if he asked them to transfer school that would be better I know it's difficult to ask for help but when u have someone u can trust just do it