Is this backbiting considered permissible, given that I spoke about an oppressor? And if it is not permissible, should I apologize to the teacher?

If backbiting is for a legitimate purpose—such as changing an evil—then it is not forbidden. Otherwise, it is forbidden backbiting, and repentance from it is obligatory by ceasing, regretting, and resolving not to return to it. If the backbiting reached the teacher and she became angry, then it is obligatory to seek her forgiveness and apologize to her.