There's a legal category, I forget the Chinese for it, that means something like "student worker". I haven't looked into the details.
As to whether or not it's sex discrimination, there are Gender Equality committees you can contact for information and complaints. There's also an entire law devoted to the subject, the Act of Gender Equality In Employment (also translated by another title sometimes). Basically it states the obvious, no gender discrimination, and goes on to establish minimum standards for various gender related issues like menstrual leave, as well as some enforcement measures.
If "marketing" in this case means starring in ads, it's not hard to see a justification there, like in any artistic project. If it's office work, it's much harder to see a justification.