What is the ruling on sitting with a teacher who utters words of disbelief to students, while inwardly disapproving of that?

Denial with the heart is beneficial when the listener's situation differs from that of the speaker, so he is not like him in the ruling of disbelief. However, the sin of listening to disbelief without denouncing it verbally remains, if one is able to do so, or if one is able to leave the gathering.