What is the ruling on a male teacher sitting with my female relative, who is eleven years old, without a mahram and next to her? And is it obligatory for me to denounce them?

A girl who has reached the age of eleven is, in her dealings with a non-mahram man, subject to the same rulings as an adult woman. Thus, it is forbidden to be in seclusion with her, to touch her, or to look at her with suspicion. The criterion for unlawful seclusion (khalwa) is any gathering where suspicion is not ordinarily safeguarded against. Therefore, it is not permissible for this girl to sit with a non-mahram man in the manner mentioned, and this must be denounced with wisdom and good admonition.