What is the ruling on wearing a wig to cover the hair instead of the khimar or hijab, which are prohibited in universities?

Wigs are permissible according to the Hanafis and Shafi'is for a married woman with her husband's permission. They are permissible according to the Hanbalis when there is a need, and permissible according to the Malikis because they are placed on the head. The preponderant opinion is that there is no harm in them for the husband's sake. Covering a woman's head is obligatory, and her head is part of her 'awrah (intimate parts) which, by consensus, must not be uncovered in front of non-mahram men. Wigs can be worn by girls in the circumstances mentioned in the question if they find no other solution and their need for study is essential.