If all her students performed well, this mistress was guaranteed a hefty bonus. All her students were doing well except this loser. He was the one standing between her and that hefty check. She could not afford to lose it so she made him work extra hard by making him eat her shit and promising to do worse to him should he not work hard and improve his grades very soon.
This mistress is cruel. There is no question about it. She likes to humiliate and degrade anyone who messes with her. And this guy accidentally messed with her. She did not care whether it was accidental or not. He had crossed her path and had to pay for it. She used him as her human toilet. She pooped onto his face and made him eat the shit. He had no choice but to do as told.