Is it permissible for a female student to go to the university to eat discounted food in the cafeteria on her days off, or after she has finished her practical work and before the time of her defense?

There is no harm in students benefiting from university discounts, as long as university regulations do not prohibit it, whether the permission is explicit or customary. What is known by custom is like what is stipulated by condition, and customary permission is like verbal permission.