I think that hajun was wearing different clothes to meet up with the person he was texting on the phone with at the end of chapter 19 and when he finished talking to that person he might have changed fast enough to arrive to the classroom he and danwoo shared without any suspicion from danwoo.
It's hard to point out the difference in them twins if the art is just like a copy paste of the ml.and I'm intrigued on what's gonna happen in the furure
I think that hajun was wearing different clothes to meet up with the person he was texting on the phone with at the end of chapter 19 and when he finished talking to that person he might have changed fast enough to arrive to the classroom he and danwoo shared without any suspicion from danwoo.