saegye is just bringing up the letter as an excuse to make sure jinsol does attend school the next day. i think jinsol has quite the sentimental attachment to the letter because the simple fact that she replied to his letter (i guess) implies that she still loves and accepts him for who he is, and i like to think that jinsol keeps the letter as a reminder to himself to be careful
shieun came up with the plan of exposing the existence of dares because she knows that by doing so, the haram case will be reopened, therefore exposing shigyoo’s lies and, as a result, be automatically out of shieun’s way as director.
+ the letter’s contents is on chapter 12
What's on that letter that's important?
Why does the long hair friend want to expose dares?