Is the niqab obligatory, or is the khimar sufficient, and what should I do if the niqab prevents me from teaching?

A woman must cover her face in front of foreign men, because the face is the center of beauty and the source of temptation. If a woman is compelled to work and cannot do so without uncovering her face, and she finds an alternative, then she must leave that job. However, if there is a necessity, such as an obligatory expense, and she has no one to support her, it is permissible for her to uncover her face due to necessity, while avoiding people's gazes and adhering to the niqab if possible. The same applies if Muslims are in need of her beneficial knowledge which no one else can provide.