When does one have to leave a gathering in which religion is reviled or mocked? And is going to places of vice for another purpose, while disavowing the vice in one's heart, equivalent in sin to those who commit it? And must one leave the classroom when that happens?

Sitting with those who mock God's verses is forbidden, but it is not disbelief if one dislikes the reprehensible act in their heart. However, one sins if they are able to leave them and do not. If those who mock cease their discourse concerning God's verses, then it is permissible to sit with them, as Allah says: "until they engage in a discourse other than it." It is not permissible to sit in a place of amusement where God's verses are mocked except with denunciation. If you are unable to change the situation, then it is not permissible for you to sit in a way that you hear and witness, as Allah says: "And He has already revealed to you in the Book that when you hear the verses of Allah being denied and ridiculed, do not sit with them until they engage in a discourse other than it." However, if you would be harmed by leaving the gathering, such as in a classroom, then you are not obligated to leave, but you are obligated to denounce the evil according to your ability. If your denunciation is not accepted, then you are obligated to denounce the evil in your heart and detest it, and there is no sin upon you for remaining.