For the guy:
For the first scene, we wanted to show him as a mature businessman who knows what he's doing. He's in formal attire to show him as a confident, different person in a way (to show that time has changed and with time, he's matured).
For the second and third scenes, it's a flashback so we show the guy as an immature person who isn't really bothered by time (like the shot where he just stares at his pen like he's waiting for something to happen). He's a laidback stereotypical late teen to early twenties guy who seems like he wants to just to live his best life.
For the girl:
For the first scene: We show the girl as suspicious and wearing all black to symbolise mystery and POSSIBLY death. We don't know any specific details because at this point we don't want to give away her intentions, whatever they are.
For the second and third scenes, she's in a rush, ready to tackle her dreams. She comes back to get her books and makes sure she isn't forgetting anything. Unlike the male lead, she's careful and seems like she wants everything to go to plan (she's career-focused).