A classroom aboard the USS Enterprise-D.
A classroom is a place where children are taught.
Aboard the Galaxy-class starship, there were several small schools of varying sizes located throughout the ship, ranging from actual classrooms (TNG: "When The Bough Breaks") to specialized workshops. (TNG: "Imaginary Friend", "Rascals", "Masks")
Keiko O'Brien opened a school on Deep Space 9 in 2369; for this purpose her husband converted a room on the promenade into a classroom. (DS9: "A Man Alone")
DS9's classroom was damaged by a bomb later that year, after Vedek Winn Adami started protesting against the secular subjects taught by Keiko. (DS9: "In the Hands of the Prophets")