ANSWER:

You must demand that one of the female observers checks your identity. If there were no females – which is highly doubtful – you can show your face for necessity as one does at an airport. 

Where I live, even if I go to an all women’s college sometimes during an examination there are male invigilators in the room to keep an eye on the students. Before entering the exam hall, for identification purposes I would be asked to remove my niqab and would only be allowed my hijab. Is this okay as I have no choice but to uncover my face during that period of time.