Does smiling at a teacher who mocks one's appearance and academic level constitute approval of her action, or should one denounce it due to weakness of faith and refrain from smiling?

It is not permissible for a Muslim to mock another Muslim, due to the Almighty's saying: "O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them; nor let women ridicule [other] women; perhaps they may be better than them," and the Prophet's (peace be upon him) saying: "It is enough evil for a person to despise his Muslim brother." The sin of ridiculing a Muslim in their appearance and form primarily involves the rights of the individual. So, if the wronged person brings their case to the judiciary and then forgives, the punishment is waived. As for ridiculing Allah's creation and formation, admonition is necessary in that case.