Is the student considered sinful for backbiting about her teachers to her mother after the mother saw them at the parents' meeting, even though the mother did not know them previously?

Mentioning female teachers by what they dislike, specifying their names, is backbiting (gheebah), and one must repent to Allah for it. Imam An-Nawawi explained the Hadith of Umm Zar‘, clarifying that forbidden backbiting is mentioning a specific person or group by name with something they dislike. However, if they are unknown, it is not considered backbiting according to some scholars, as mentioned by Al-Mazari and Qadi Iyad.