Am I sinful if I laugh at my friend's sarcastic comments about the teacher?

It is not permissible to mock a Muslim; this is among the forbidden acts that Allah and His Messenger (peace be upon him) have prohibited, as Allah the Almighty says: "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 do not insult one another and do not call each other by [offensive] nicknames. Wretched is the name of disobedience after [one's] faith. And whoever does not repent - then it is those who are the wrongdoers." Mocking a female teacher is even more heinous because it violates her right to veneration and respect. Laughing at the words of the mocker is considered participation in the sin, and whoever falls into this must repent to Allah, apologize to the teacher, and advise the friend to repent.