How easy is it to determine the identity of women who wear veils?

I know there is a lot of controversy about Moslem women in western countries wearing veils, but I ask this as a strictly informational question. Suppose that all the female students in a class wear veils. How easily could the teacher identify them and how easily can they identify one another?

I would think that from a distance it would be hard, but what about up close? There are of course clues to a woman’s identity other than her eyes, but can you train yourself to recognize people just from their eyes?