Is it permissible for a niqabi student who suffers from headaches that prevent her from concentrating and worsen with wearing the niqab, to remove the niqab and suffice with covering her entire face in a way that does not strain her head, knowing that her specialization requires removing the niqab during training in the hospital?

Covering the face is obligatory. It is permissible to cover it while exposing the eyes for vision and writing. There is no need to tie the head if it is already covered. Uncovering the entire face is not permissible except out of necessity or need. Uncovering the face during training in the presence of non-mahram men is not permissible. One must try to convince the administration to accept the training with the face covered. However, if they insist on refusal and there is a dire need for the training, then there is no harm in uncovering the face due to need, while avoiding direct interaction with men as much as possible, not wearing makeup, and returning to covering the face once the need is no longer present.