The class monitor guy pushed her and Miss Zhang, the teacher, saw it but is lying to cover for him. It would seem there is some incentive to keep this top student from getting punished, prehaps her pay is refected on her student's performance. The teacher attempts to gaslight Jiajia, the victim, causing her to doubt her memory but also realizes there is a strong possiblity she isn't going to remember anything after this moment so she decided to tell the teacher her own lie to make her believe the class monitor intended to push the teacher and turned her against him.
