From previous episodes (also told in flashback), Rob was a Staff Sergeant (three chevrons and 1 rocker) when he began to romance Laura (the USO dancer) (Season One: Oh How We Met the Night that We Danced) and again when they were getting married (Season Two: The Attempted Marriage). But in this episode, Rob was a mere Sergeant (three chevrons and no rockers). It is highly unlikely that he was demoted briefly and then promoted again; it is more likely that they forgot from the original rank they had previously claimed for him.
If Clark Rice had already attended law school, he most likely would have been an officer instead of an enlisted man.