A sophomore was a cadet at Starfleet Academy in his second year.
During his sophomore year, Jean-Luc Picard was assigned to training at Morikin VII. (TNG: "Tapestry")
In his sophomore year, Benjamin Sisko was posted for his field study at Starbase 137. (DS9: "The Ascent")
When Jadzia Dax was a sophomore, Leslie Wong was an instructor at Starfleet Academy. (DS9: "In the Pale Moonlight".
In the first draft script of the episode, Dax told Benjamin Sisko that she had been a plebe when Wong had been an instructor at the Academy. However, in the final version of the episode, this reference to "plebe" was instead changed to refer to Dax having been a sophomore.