Security guards, technicians, yeomen, and specialists have all been shown to be enlisted personnel. I assumed Keenser was enlisted because Scotty treats him like a dogsbody and he never gives orders but the IDW comic stated him to be a lieutenant - i.e. quite a senior officer! Bonkers.
Jaylah would work well as a security guard IMO. She doesn't need to be an officer. However, Starfleet Academy may also offer a basic training course in Federation law, technology, history sociology, and stellar cartography in addition to the specialism the enlisted crewman wants to study. Graduates would be officers but I don't see why we have to assume that enlisted personnel would not be able to study at the same establishment for a time.
