What is the ruling on a teacher being in seclusion with a female student during a private lesson?

It is not permissible for a man to be alone with a woman or a pubescent girl who is desired, because the Prophet, peace and blessings be upon him, said: "No man is alone with a woman but the third of them is Satan," and "No man is alone with a woman unless there is a mahram with her." The criterion is their being together in a place where they are safe from the sight of others. To ensure not falling into what Allah has forbidden, someone must sit with them during the lesson.