I believe that they were in college and not high school. He refers to Yi Jie as "Professor" which is a common term for college in these dramas. That's why they were able to get married within the time he graduated. Plus, when they started dating, he mentioned he was of adult age. :)
I will be referring to Jungwoon as he so it is less confusing to follow. The movie is Jungwoon talking to the female he wishes he was. This is why he told her "This is all of your fault." In the laundry mat. The tunnel is where Jungwoon would meet the light if he were to kill himself. The female tells him how pretty they are and convinces him to stay and become who he truly wants to be. That's why he wakes up on the beach.