Yet we have not heard of anybody in Starfleet graduating at higher than Ensign rank. Supposedly, in Roddenberry's "astronaut" fleet, everybody is an academician and deserving of an insta-promotion, so nobody gets it.
Sure, we have seen people hold the rank of Lt(jg) at an early stage of their careers. Saavik, Bashir and the like could still have graduated as Ensigns and soon thereafter received promotions since, as pointed out, Starfleet appears to dish out promotions as an alternative to commendations that in today's militaries would result in medals and other decorations instead.
The new movies cast some doubt here, as it would appear McCoy is a LtCmdr only three years after boarding a shuttle to Starfleet for the very first time. In TOS, we could speculate that he had received orderly Starfleet training in addition to his medical studies without
immediate compensation in rank; in nuTrek, it seems the medical studies come first, and three years (or much less - in terms of movie evidence, McCoy may well have graduated long before Kirk finished his third try at Kobayashi Maru
) in Starfleet training after that is enough for an insta-promotion in today's style.