What should a girl do who believes, along with her father, that the niqab is obligatory, but he prevents her from wearing it out of fear for her safety due to the security situation in their country, which they only visit rarely, knowing that she is a university student in the medical field and will begin her mixed-gender practical training period?

As long as you believe in the obligation of covering the face from non-mahram men, it is obligatory for you to cover it when going out to university or elsewhere. Your father does not have the right to prevent you from doing so, as long as no harm results from it. If your father insists on preventing you, and you fear that he might prevent you from going out, then you can cover your face without informing him. If this becomes difficult for you, then it is not permissible for you to go out except for a need or necessity.