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.
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.