What is the ruling on working without the knowledge of one's parents, as they completely disapprove?

The basic principle is that a woman's staying in her home is better for her and more protective against temptation, in compliance with Allah's command to the wives of the Prophet (peace be upon him): "And abide in your houses." However, a woman is permitted to work, provided she adheres to legitimate Islamic guidelines, including: obtaining permission from her husband or guardian, unless she is in need of financial support, in which case it is permissible for her to go out to earn a living. The financial support of a daughter is obligatory upon her father until she marries and her husband consummates the marriage with her.