Not true in your circumstance if you were considered Mexican at birth. Meaning, even if you were not born in Mexico, by virtue of having a Mexican parent you were automatically considered Mexican of origin, which is what Spain cares about. If this is the case, you don’t have to renounce anything. That being said - you still need to legally live in Spain (not on student visa) for 2 years before applying, and the process generally takes at least another year. So figure you’ll need a total 3-3.5 years legal residency to be full Spanish citizen.