It's rude to enjoy the misfortune of a classmate.
Ken was in class when he saw Sally being fussed at by the teacher.
Ken wants to do his work.
Ken ignores what is going on with Sally and does his classwork.
Ken laughs at Sally while he continues doing his classwork.
Ken makes an A in the class because he stays focused on his own work.
Ken's teacher fusses at him for not minding his own business.