Is describing the behavior of a teacher as "unfit to be a teacher" considered backbiting (gheebah), and how can one repent from that?

The fundamental principle is that it is forbidden to mention a Muslim with anything he dislikes. Exceptions are made only when it is the sole means to achieve a legitimate purpose, such as complaining about an oppressor, warning against evil people, or seeking assistance to change a wrong. The backbiting of someone who openly commits sin and evil is not blameworthy. Backbiting is permissible for every legitimate religious purpose when it is the only way to achieve it, such as seeking redress, asking for a fatwa, judicial proceedings, and warning against evil. If we are hesitant about the existence of a reason for an exception, we remain with the original principle, as is the case with saying "someone is unqualified to be a teacher," because no benefit results from it. It is permissible to mention about an open sinner what he publicizes, but not other things unless there is another reason. As for repentance from backbiting, some scholars hold that sincere repentance, seeking forgiveness for the person one backbit, supplicating for him, and mentioning him in a good way, are sufficient.