You had a boyfriend at school (Changbin), Changbin was a popular student at your school, while there was bangchan, bangchan was a student who gets straight A's and was diligent

After class, you and changbin split up because he had to get something from his locker

While Changbin was walking to his locker, Bangchan pushes him to an empty classroom

Changbin:what do you want, Man?

Bangchan:your girlfriend wants to be my girlfriend, she be calling me telling me 'bout you

Changbin:where's your proof?