Lady Phoenix is a BBW. She knows it and she is ok with it. But she was pissed when she found out that this guy had been making fun of her weight. She was not going to look away and let him continue doing what he had done. So she used her pee to punish him. He had to drink it through a funnel and he had no choice but to do it.
Lady Phoenix felt that her classmate had to be punished and she chose to do it with her pee. The mistress even invited her fellow BBW mistress and the two of them went ahead to force the classmate to drink their pee. He was shocked at what the mistresses did to him but he did not have a choice but to do what the mistresses wanted. And he never angered them again.