Mendon appears to have not only been accepted into the academy, but also made the real rank of ensign before Crusher was even given the proper uniform.
Only if we assume that Mendon was a Starfleet Ensign rather than a Benzar Defense and Occasional Conquest Force one.
We never learned the rules of this exchange student game. Riker wore his own uniform to a Klingon ship, and Kurn later wore his to a Starfleet vessel, but regardless of his choice of attire, Mendon was still supposed to be part of the exchange program - Riker clearly says so. This heavily suggests he was not
in Starfleet, as his exchange assignment was aboard a Starfleet ship!
Sure, Wesley seems
to be confused specifically because he thinks the Benzite must be a Starfleet Academy graduate even though at the same time he cannot be. But what does Wesley know? Mendon never confirms he would have graduated from the SF Academy, or even attended it.