Is it permissible for my fiancée to wear the niqab and remove it upon entering school, knowing that the place is devoid of men, and would she be sinful if a man saw her in this state?

If the situation is as the questioner described, that his fiancée uncovers her face in a children's school where there are no men, then there is no harm in that. If a man sees her unintentionally, there is no sin. However, she must cover her face if she knows that someone is looking at her, and whoever sees her must lower his gaze. But if the children are adolescents or have become aware of women's matters, then she must observe hijab from them and not uncover her face in front of them.