"My student lives in the city."
Translation:Discipula mea in urbe habitat.
Mea can be a feminine form, but it can also be neuter. You also have to consider the noun it is with.
discipula mea -> my student: mea is feminine since it agrees with discipula (which is feminine)
vestimenta mea -> my clothes: mea is neuter here since vestimenta is neuter.
Do you mean dropping the preposition "in" and using ablative case, as for city names? That does not seem to be a good idea for the word city, urbe.
Wikipeady reads: "There are a few nouns that use the locative instead of a preposition: domus becomes domī (at home), rūs becomes rūrī (in the country), humus becomes humī (on the ground), militia becomes militiae (in military service, in the field), and focus becomes focī (at the hearth; at the center of the community). " https://en.wikipedia.org/wiki/Locative_case